COMOCNOPSINST AVN 30 JAN 2006

Transcription

1 COMOCNOPSINST AVN 30 JAN 2006 COMOCNOPS INSTRUCTION From: Commander, Oceanographic Operations Subj: SURFACE WEATHER OBSERVATION PROCEDURES Ref: (a) NAVMETOCCOMINST L (b) Office of the Federal Coordinator for Meteorological Services, Federal Meteorological Handbook No.1, Surface Weather Observations and Reports (c) 49 U.S. CODE 44720, Meteorological Services (d) World Meteorological Organization, Manual on Codes Volume II, Regional Codes and National Coding Practices (e) NAVMETOCCOMINST K (f) U.S. Marine Corps Order Encl: (1) USN/USMC Surface Weather Observation Procedures Manual 1. Purpose. Surface weather observations are fundamental to all meteorological services. They are the basis for all forecasts and warnings. The purpose of enclosure (1) is to provide all USN and USMC METOC activities with detailed instructions for observing, recording, and encoding surface weather observations for shore stations. This instruction includes procedures for both automatic and manual observations, as well as procedures for augmentation and back-up of automatic observations. This instruction incorporates World Meteorological Organization (WMO) observation requirements for encoding the METAR code. 2. Cancellation. NAVMETOCCOMINST Discussion. Reference (a) directs all USN and USMC METOC activities supporting aviation operations to conduct a surface observation program. Reference (b) provides general procedures for all U.S. governmental agencies engaged in weather observation activities and directs those agencies to develop observation program procedures. Reference (c) states that the FAA shall coordinate meteorological requirements in the United States to maintain standard observations, to promote efficient use of facilities, and to avoid duplication of services unless the duplication tends to promote the safety and efficiency of air navigation. Reference (d) details the METAR and SPECI codes, which are used by all US governmental departments engaged in weather observation activities. The enclosed manual provides the specific instructions for Navy and USMC personnel as derived from the guidance contained in references (b) through (d). The accurate and timely submission of environmental observations are critical for support of naval operations. References (e) and (f) define observer qualification criteria.

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3 COMOCNOPSINST Distribution (Cont.): FD2 NAVMETOCPRODEVCOM (20 Copies) FD3 FLENUMMETOCCEN (5 Copies) FD4 Ice, Meteorology and Oceanography Center (10 Copies) FD5 Meteorology and Oceanography Centers (5 Copies) FF5 Safety Center FF42 Postgraduate School FKAIB PEO C4I & SPACE (PMW-180) FKR6B Air Warfare Center Weapons Division (Point Mugu only) FR3 Air Station COMNAVRESFOR FT1 Chief of Naval Education and Training (5 Copies) FT2 Air Training Center FT10 Aviation Schools Command FT13 Air Technical Training Center FT15 Technical Training Unit Keesler (100 Copies) FT 17 Naval Education Technical Training FT24 Fleet Training Center FT31 Training Center (Orlando only (ATTN.- QM-A)) FT78 Education and Training Program Management Support Activity (Tech. Library and AG/311 only) V3 Marine Corps Air Bases (ATTN: METOC Officer) V4 Marine Corps Air Facility (ATTN: METOC Officer) V5 Marine Corps Air Station (ATTN: METOC Officer) V12 Marine Corps Combat Development Command V25 Marine Corps Air-Ground Combat Center (ATTN: METOC Officer) MARCORDET Keesler AFB (20 Copies) Federal Aviation Administration (ATTN: Weather Services) Office of the Federal Coordinator for Meteorological Services and Supporting Research (OFCM), 8455 Colesville Road, Suite 1500, Silver Spring, MD HQ, AFWA, Scott AFB, IL

4 Commander Oceanographic Operations Stennis Space Center, Mississippi COMOCNOPSINST December 2005 SURFACE WEATHER OBSERVATION PROCEDURES MANUAL RET RU This publication is required for observation and dissemination of Surface Weather Observations and has limited distribution to U.S. Government agencies and their contractors. Any queries as to content of this document should be referred to Commander Oceanographic Operations AVN Stennis Space Center, Mississippi (Enclosure 1)

5 TABLE OF CONTENTS TABLE OF CONTENTS... ii Record of Changes.....xv CHAPTER 1- INTRODUCTION 1.1 Purpose Relationship to Federal Meteorological Handbook No.1 and other Instructions Importance of Observations Applicability and Practices Abbreviations and Acronyms Changes to this Manual Unforeseen Requirements Observer Responsibility Instrument Evaluations OCONUS Reporting Recommendations CHAPTER 2 OBSERVATION PROGRAM GENERAL PROCEDURES 2.1 Definitions Weather Observing Station, METOC Office or Activity Observed Surface Weather Observation Manual Observation Automated Observation Augmented Observation Backup Coordinated Universal Time (UTC) Local Standard Time (LST) Aircraft Mishap Types of Stations Automated Station Augmented Station Manual Station Towered Station Navy METOC Service Standards for Observing Level D Level C Level B Level A Determination of the Level Of Service Requirement for Observations Observations During non Augmented Hours Observing Locations Primary with ASOS Primary for Manual and Backup ii

6 2.7 Control Tower Observations and Weather Station Actions Required ATC Personnel Actions Required Weather Activity Personnel Actions Types of Surface Weather Observations and Codes METAR SPECI Synoptic Observations Table 2-1 Criteria for SPECI through Time Procedures Actual Date and Time of Observation Standard Time of Observation Time Checks Order of Reporting Rounding Figures Night Vision Adjustment Certifications and Standards Surface Weather Observer Certification Tower Visibility Observer Certification Equipment Certifications Automated Surface Observing System (ASOS) Certification Algorithm Certification Documentation and Records Manual Retention and Archive Electronic Archive and Records Station Information File Sensors and Instruments Primary Sensors Required Backup Instruments Calibration and Maintenance Observing Aids for Visibility Visibility Check Point Diagrams Selecting Markers Panoramic Photo Periodic Review of Visibility Markers Control Tower Visibility Aids Observation Quality Control Dissemination Local Long-line or Distant, Network Corrections to Previously Disseminated Reports Delayed Reports CHAPTER 3 ASOS AUTOMATED OBSERVATIONS 3.1 Why Automate Weather Observations? ASOS User s Guide and the ASOS Ready Reference Guide ASOS Description ASOS Sensor Group Figure 3-1 ASOS Sensor Group iii

7 3.3.2 Sensor Location Figure 3-2 ASOS Acquisition Control Unit Acquisition Control Unit (ACU) Operator Interface Device (OID) Video Display Unit (VDU) Available ASOS Displays METAR Elements Auxiliary Data Maintenance Data Summary Data The ASOS Observation Process The Objective Elements The Subjective Elements ASOS Techniques ASOS Algorithms Human vs. ASOS METAR Quality Control of ASOS Observations Level Level Level Ambient and Dew Point Temperature Observation Ambient and Dew Point Temperature Sensors Figure 3-3 Temperature and Dew Point Sensors Figure 3-4 New ASOS Dry HUMICAP Dew Point Sensor Temperature and Dew Point Primary Data and Algorithm Other Temperature and Dew Point Data ASOS Temperature/Dew Point Strengths and Limitations Wind Observations Wind Sensors Figure 3-5 ASOS Wind Sensor Wind Algorithm and Primary Data Other Wind Data ASOS Wind Strengths and Limitations Pressure Observations Pressure Sensor Pressure Algorithm and Primary Data Other Pressure Data ASOS Pressure Strengths and Limitations Precipitation Accumulation Observations Precipitation Gauge Figure 3-6 Heated Tipping Bucket The Precipitation Accumulation Algorithm and Primary Data Other Precipitation Amount Data ASOS Precipitation Accumulation Strengths and Limitations Sky Condition Observations Cloud Height Sensor Figure 3-7 Ceilometer Basic Sky Condition Algorithm iv

8 Determining the Layers Determining the Amount Table 3-1 ASOS Cloud Amount Report-Percent of Sky Obscurations Variable Ceiling Special Report Generation ASOS Sky Condition Strengths and Limitations Visibility Observations Visibility Sensor Figure 3-8 Visibility Sensor Visibility Algorithm and Primary Data Special Report Generation Variable Visibility ASOS Visibility Strengths and Limitations Present Weather and Obscuration Observations Precipitation Identification Sensor Figure 3-9 Precipitation Identification Sensor Precipitation Identification Algorithm and Primary Data Other Precipitation Data ASOS Precipitation Identification Strengths and Limitations Single Site Lightning Sensor Figure 3-10 Single Site Lightning Sensor Single Site Lightning Sensor Algorithm ASOS Lightning Sensor Strengths and Limitations Freezing Precipitation Sensor Figure 3-11 Freezing Precipitation Sensor Freezing Rain Algorithm ASOS Freezing Rain Strengths and Limitations Obscuration Algorithm ASOS Obscuration Algorithm Strengths and Limitations Present Weather Algorithm Processing Matrix Table 3-2 Present Weather Algorithm Processing Matrix ASOS Observation Dissemination ASOS Data Archive and Recall Data Not Provided by ASOS Planned Improvements CHAPTER 4 - AUGMENTATION OF ASOS OBSERVATIONS 4.1 Augmentation Requirements When Who What Specific Element Augmentations Tornadic Activity Thunderstorms Hail Volcanic Ash Virga v

9 4.2.6 Freezing Rain and Freezing Drizzle Ice Pellets Snow Pellets, Snow Grains, and Ice Crystals Snow Depth Remarks Lightning not at the Station Dust, Sand, Spray, and Smoke (obstructions not sensed by ASOS) Cloud Layers above 12,000FT Variable Sky Condition Visibility values of 1/8 and 1/16 SM Sector Visibility Significant Clouds Runway Condition Additional Remarks Tower Visibility Augmentations Basic Operator Interface Device (OID) Operations Figure 4-1 OID 1-Minute Edit Screen Sign-On and Sign-Off Procedures Table 4-1 Sign-On and Sign-Off Procedures Sign On/Sign Off When Entering SPECI Special Keys Table 4-2 Special Function Keys Alarms CHAPTER 5 - BACKUP OBSERVATION PROCEDURES FOR ASOS 5.1 Backup Equipment Backup Actions ASOS Partial Failure ASOS Communications Failure ASOS Total Failure Suspension of Augmented Observations Observer Priorities Recording of Backup Observations CHAPTER 6 - MANUAL OBSERVATIONS GENERAL PROCEDURES 6.1 The Observational Process Weather Watch and Observer Responsibility Types of Aviation Weather Reports Aviation Weather Reports - METAR Aviation Selected Special Weather Reports SPECI Encoding of the Observations Procedures and Assumptions CHAPTER 7 - WIND OBSERVATIONS 7.1 Definitions Direction of Wind Speed of Wind Gusts Squall vi

10 7.1.5 Magnetic Variation Peak Wind Speed Variable Wind Direction Wind Shift Wind Evaluation Wind Direction Wind Speed Table 7-1 Estimating Wind Speed Beaufort Scale Gusts Squalls Peak Wind Speed Wind Shifts Conversion of True to Magnetic Winds To Convert from True to Magnetic Direction To Convert form Magnetic to True Direction Priority of Instruments Conversion of Wind Speed Table 7-2 Conversion of Miles per Hour to Knots Table 7-3 Conversion of Knots to Miles per Hour CHAPTER 8 - VISIBILITY OBSERVATIONS 8.1 Definitions Prevailing Visibility Surface Visibility Tower Visibility Sector Visibility Variable Prevailing Visibility Visibility Markers Visibility Observation Standards and Requirements Location of Observations Unit of Measure Table 8-1 Reportable Visibility Values Observing Aids for Visibility Figure 8-1 Visibility Check Point Diagram Dark Adaptation Control Tower Visibility Observations Evaluating Visibility Determining Prevailing Visibility Sector Visibility Variable Prevailing Visibility CHAPTER 9 - WEATHER PHENOMENA OBSERVATIONS 9.1 Precipitation Drizzle (DZ) Rain (RA) Freezing Drizzle (FZDZ) Freezing Rain (FZRA) Snow (SN) vii

11 9.1.6 Snow Grains (SG) Small Hail or Snow Pellets (GS) Ice Pellets (PL) Ice Crystals (IC) Hail (GR) Unknown Precipitation Obscurations Fog (FG) Mist (BR) Freezing Fog (FZFG) Blowing Snow (BLSN) Spray (PY) Haze (HZ) Smoke (FU) Widespread Dust (DU) Blowing Dust (BLDU) Blowing Sand (BLSA) Volcanic Ash (VA) Other Required Reportable Phenomena Thunderstorm (TS) Lightning Tornadic Activity Volcanic Eruption Virga Well-developed Dust/Sand Whirl (PO) Sandstorm (SS) Duststorm (DS) Qualifiers and Descriptors Intensity Showery Continuous Intermittent Proximity Shallow (MI) Partial (PR) Patchy (BC) Low Drifting (DR) Blowing (BL) Other Phenomena and Miscellaneous Terms Glaze Rime Water Equivalent Core Sample Snowboard Rainbow Halo Corona Crepuscular Rays viii

12 9.6 Evaluating Precipitation Times Character Intensity Table 9-1 Estimating Intensity of Rain or Freezing Rain Table 9-2 Estimating Intensity of Ice Pellets Table 9-3 Intensity of Rain or Ice Pellets Based on Rate of Fall Table 9-4 Intensity of Snow and Drizzle Based on Visibility Amount Table 9-5 Precision of Precipitation Measurements Determining Water Equivalent for Frozen/Freezing Precipitation Table 9-6 New Snowfall to Estimated Melt-water Conversion Measurement of Solid Precipitation Evaluating Obscurations Determining Type of Obscuration Haze vs. Mist Table 9-7 Guide to Selection of Obscuring Phenomena Obscuring Qualifiers Volcanic Ash Evaluating Thunderstorms Beginning and Ending of a thunderstorm Location Movement Evaluating Lightning Table 9-8 Type and Frequency of Lightning Evaluating Tornadic Activity Begins Ends Other Details Evaluating Volcanic Eruption CHAPTER 10 - SKY CONDITION OBSERVATIONS 10.1 Definitions Field Elevation Celestial Dome Horizon Cloud Obscuring Phenomena Vertical Visibility Layer Layer Amount Layer Height Layer Opacity Summation Principle Sky Cover Sky Cover Classification Ceiling Variable Ceiling ix

13 Variable Sky Condition Total Amount Cloud Movement Sky Cover Evaluation Evaluating Cloud Height Table 10-1 Cloud Height Reportable Values Table 10-2 Convective Cloud-Base Height Table Evaluating Layers Evaluating Amount Table 10-3 Sky Cover Amount Evaluation Determining Sky Cover Classification Table 10-4 Sky Cover Classification Determination of Ceiling Table 10-5 Criteria for Variable Ceiling Significant Clouds Examples of Sky Condition Evaluation Table 10-6 Sky Cover Evaluation Examples CHAPTER 11 TEMPERATURE AND DEW POINT OBSERVATIONS 11.1 Definitions Ambient Temperature Dry Bulb Temperature Wet Bulb Temperature Wet Bulb Depression Dew Point Relative Humidity Heat Index Wet Bulb Globe Temperature Wind Chill Index Psychrometer Hygrothermometer Psychrometric Calculator or Computer Psychrometric Tables Evaluating Temperature and Dew Point Units of Measure Observation Period Priority of Instruments Use of Psychrometers Use of Portable Hygrothermometers Dew Point Equal to or Exceeding Air Temperature Use of the Psychrometric Calculator or Computer Heat Index and Wind Chill Determining Heat Index Values Table 11-1 Heat Index Table Determining Wet Bulb Globe Temperature Determining Wind Chill Values Table 11-2 Wind Chill Table Temperature Conversion x

14 Figure 11-3 Temperature Conversion Fahrenheit to Celsius , 11-8 Figure 11-4 Temperature Conversion Celsius to Fahrenheit CHAPTER 12 - PRESSURE OBSERVATIONS 12.1 Definitions Field Elevation Station Elevation Atmospheric Pressure Barometric Pressure Station Pressure Sea Level Pressure Altimeter setting Standard Atmosphere Pressure Altitude Density Altitude Precision Aneroid Barometer Removal Correction Instrument Correction Pressure Reduction Computer or Calculator Pressure Reduction Constant Pressure Change Pressure Characteristic Pressure Tendency Pressure Falling Rapidly Pressure Rising Rapidly Pressure Unsteady Determining Station Pressure Priority of Instruments Rounding Pressure Values ASOS Pressure Readings Determining Station Pressure from the Aneroid Barometer Calculating Other Pressure Readings Sea Level Pressure Calculation Altimeter Setting Calculation Pressure Altitude Calculation Table 12-1 Pressure Altitude/Standard Atmosphere Table through Density Altitude Calculation Determining Pressure Tendency Pressure Change Calculation Determining Pressure Characteristic Table 12-2 Pressure Characteristics Evaluating Significant Pressure Changes Pressure Falling Rapidly Pressure Rising Rapidly CHAPTER 13 - CODING AND DISSEMINATION 13.1 Definitions Coding or Encoding xi

15 Dissemination Long-line Dissemination Local Dissemination FIBI Filing Time COR METAR/SPECI Codes Table 13-1 Content and Format of METAR and SPECI Body Remarks Missing Data Coding the Body of the METAR/SPECI Type of Report Station Identifier Date and Time of Report Report Modifier Wind Group Visibility Group Present Weather Group Table 13-2 Present Weather Precedence Sky Condition Group Temperature and Dew Point Group Altimeter Group Remarks Manual, Augmented and Plain Language Volcanic Eruptions Tornadic Activity Type of Automated Station Peak Wind Wind Shift Tower Visibility or Surface Visibility Variable Prevailing Visibility Sector Visibility Lightning Beginning and Ending of Precipitation and Thunderstorms Thunderstorm Location Hailstone Size Virga Variable Ceiling Obscurations Variable Sky Condition Significant Cloud Types Pressure Rising or Falling Rapidly Sea Level Pressure Aircraft Mishap Snow Increasing Rapidly Runway Condition Table 13-3 Runway Surface Condition Other Significant Information xii

16 FIRST and LAST Remarks Additive Data Hourly Precipitation Amount , 3- and 6-Hourly Ice Accretion Amounts and 6-Hourly Precipitation Amounts Hour Precipitation Amount Snow Depth on Ground Water Equivalent of Snow on Ground Hourly Temperature and Dew Point Hourly Maximum Temperature Hourly Minimum Temperature Hour Maximum and Minimum Temperature Hourly Pressure Tendency Table Hour Change in Pressure Sensor Status Indicators Precipitation Identifier Inoperative Tipping Bucket Rain Gauge Inoperative Freezing Rain Sensor Inoperative Lightning Detection System Inoperative Maintenance Indicator General Dissemination Requirements Priority Data Disseminated to Tower Verification of Disseminated Data Communication Failure CHAPTER 14 - ENTRIES ON THE SURFACE WEATHER REPORT FORM 14.1 General Entry Requirements Qualification to Make Entries Writing Instrument Missing Data Late Reports Corrections Heading Summary and Climatology data Entries by Column Type of Report Time of Report Wind Direction Wind Speed Wind Gust Wind Variability Surface Visibility/Tower Visibility Present Weather Sky Condition Temperature Dew Point Temperature Altimeter Setting xiii

17 Remarks Table 14-1 Common Column 13 Remarks through Total Sky Cover Dry Bulb Temperature Wet Bulb Temperature Relative Humidity Station Pressure Sea Level Pressure Pressure Altitude Density Altitude Hourly Precipitation Observer s Initials UTC Time LST Time Report Number Maximum Temperature Minimum Temperature Precipitation Snowfall Snow Depth Active Runway Time UCT Runway Hour Maximum Temperature Hour Minimum Temperature Hour Precipitation (Water Equivalent) Hour Snowfall Snow Depth Water Equivalent Speed of Peak Wind Direction of Peak Wind Time of Peak Wind Remarks, Notes and Miscellaneous Phenomena Additional Instructions for Part Time Stations Tailoring Synoptic Data and Summary of the Day Precipitation Snowfall Summary of the Day APPENDIX A Abbreviations and Acronyms...A-1 through A-9 xiv

18 RECORD OF CHANGES CORRECTION OR CHANGE NUMBER DATE OF CHANGE DATE ENTERED BY WHOM ENTERED xv

19 CHAPTER 1- INTRODUCTION 1.1 Purpose. This manual prescribes surface weather observing standards and procedures applicable to Navy and Marine Corps activities engaged in observing and coding surface weather reports at fixed and mobile shore sites. Although these observations are used primarily in support of aviation, they are occasionally used to support a variety of other naval operations. The manual is based on agreements with and instructions issued by the World Meteorological Organization, the Federal Coordinator for Meteorological Services and the Federal Aviation Administration. It is intended to provide a framework within which an individual can identify meteorological phenomena and report the occurrence in a standardized and understandable format. 1.2 Relationship to Federal Meteorological Handbook No.1 (FMH-1) and other instructions. The FMH-1 prescribes surface weather observing standards for all U.S. federal agencies engaged in taking and reporting surface weather observations in support of aviation operations. FMH-1 also prescribes the standard reporting and coding procedures. All agencies issue additional instructions further defining their unique observation and reporting programs. This manual, like those of the other agencies, is based on the FMH-1; it defines the Department of the Navy s observations and reporting program. Federal Meteorological Handbook No. 2 provides instructions pertaining to taking and recording synoptic observations. NAVMETOCCOMINST D United States Navy, Manual for Ship s Surface Weather Observations provides similar guidance for shipboard weather observations. The Automated Surface Observing System Ready Reference Guide (ASOS) and the ASOS User s Guide provide instruction on the use of ASOS and ASOS generated data. All observing stations shall have these additional instructions. 1.3 Importance of Observations. Surface weather observations at air stations have a direct impact on aviation safety and aid in the determination of whether operations can be conducted and missions accomplished. Aircraft do not need a forecaster on-scene to land, but they do need a current observation. Likewise, the success of other military operations (such as go, no go decisions) is often dependent on an accurate description of the current weather. Whether the observation is accomplished automatically with human augmentation or completed manually, it is a vital piece of information for the pilot preparing to land. Also, these observations become a permanent official record and are the basis for climatology produced for each station. METOC personnel should never underestimate the responsibility associated with surface weather observing. 1.4 Applicability and Practices. Procedures and practices described in this manual are applicable to all U.S. Navy and U.S. Marine Corps personnel assigned to weather support activities engaged in support of aviation and other operations. Throughout this manual, the following definitions apply: a. "Shall" indicates a procedure or practice is mandatory; b. "Should" indicates a procedure or practice is recommended; c. "May" indicates a procedure or practice is optional; d. "Will" indicates futurity; it is not a requirement to be applied to practices. 1-1

20 1.5 Abbreviations and Acronyms. Appendix A contains the abbreviations and acronyms used throughout this manual. 1.6 Changes to this Manual. Due to the size, technical content and interrelationships with other agencies, changes, additions, deletions, and corrections to this manual will occur occasionally and be issued as necessary under the titles Change No. 1, 2, 3, etc. These changes are under the cognizance of the Commander, Naval Meteorology and Oceanography Command. Observing stations must enter these changes (the number, effective date, date entered and initials) manually and annotate on page XV that the change was accomplished. 1.7 Unforeseen Requirements. No set of instructions can cover all possibilities in weather reporting. Observers must use their own judgement to describe phenomena not adequately covered by specific instructions while adhering as closely as possible to this manual. Nothing in this manual should prevent personnel from reporting phenomena that would affect safety of flight. 1.8 Observer Responsibility. Personnel are expected to be alert to meteorological situations conducive to significant changes in weather conditions, and to make and disseminate weather reports as rapidly as feasible. This is especially important for phenomena that can adversely affect flight operations, such as thunderstorm activity, wind shifts, freezing precipitation, variable ceilings and visibility. Other military operations may have different weather elements that uniquely affect their accomplishment and the observer must be aware of these criteria also. Observations that meet the SPECI criteria shall be disseminated as soon as observed. 1.9 Instrument Evaluations. When there is reason to believe that the accuracy or validity of output from meteorological equipment is questionable, discontinue the use of such equipment until necessary corrective maintenance has been accomplished and utilize backup equipment and procedures OCONUS Reporting. Activities outside of the continental United States (OCONUS) have some requirements which differ from CONUS. These differences are highlighted and labeled OCONUS Recommendations. The manual contains significant changes from previous observation practices. All recommendations for improvement, whether textual, examples, displays or format are welcome. Submit all recommendations for change to the Commander, Naval Meteorology and Oceanography Command (Code N32). 1-2

21 CHAPTER 2 OBSERVATION PROGRAM GENERAL PROCEDURES This chapter provides the general procedures for the operation of a weather observing station and an observing program. It describes the various types of surface weather reports, including the Aviation Routine Weather Report (METAR) and Aviation Selected Special Weather Report (SPECI), and the content of each of these types. Also presented are general guidelines regarding augmentation and backup of automated observations. Lastly, this chapter presents guidelines on the certification of weather observers. 2.1 Definitions Weather Observing Station, METOC Office, or Activity. The general location from which weather observations are taken. The terms activity and station are used throughout this manual to indicate any weather site (permanent-type or non-fixed location) that performs a weather observing function Observed. Indicates reported weather information was determined visually by METOC personnel, or by weather sensing equipment Surface Weather Observation. A measurement or evaluation (manual, automated, or augmented) of one or more meteorological elements that describe the state of the atmosphere at the location where the observation is taken Manual Observation. Any observation for which METOC personnel observe, evaluate, prepare, record, and transmit the observation without the use of an automated observing system Automated Observation. Any observation that has been evaluated, prepared, and transmitted by a suite of electronic sensors and associated computer without human intervention Augmented Observation. Any automated observation containing data that has been modified or containing data that has been added by a human observer Backup. A method of providing meteorological data, documentation, and/or communication of a weather observation when the primary automated method is unavailable or unrepresentative. Often this is a manually taken observation transmitted via an alternative network Coordinated Universal Time (UTC). The time in the zero degree meridian time zone used on all transmitted data. All times refer to the 24-hour clock and are commonly called "ZULU" times. The times 0000 and 2359 indicate the beginning and ending of the day, respectively Local Standard Time (LST). LST is a time based on the geographic location of the facility in one of the legally established time zones of the globe. 2-1

22 Aircraft Mishap. An unplanned event or series of events (emergency) involving an aircraft that: (1) results in damage to aircraft; (2) could result in damage; or (3) results in injury, illness or death. The term "onboard" includes all interior and exterior aircraft surfaces. 2.2 Types of Stations. The generic types of stations that take surface weather observations are defined as follows: Automated Station. An activity equipped with an automated surface weather observing system that senses the elements and prepares the observation without a certified observer on duty. The observation is displayed locally and may be transmitted to other locations locally or worldwide. In CONUS and at selected overseas sites, the ASOS performs these functions. At select mobile sites, an automated weather observing system (AWOS) can sense the elements but does not have the capacity to automatically transmit the observation Augmented Station. An activity with an automated surface weather observing system that senses the elements and in some cases prepares the observation, but with a certified observer on duty capable of verifying the ASOS data and adding operationally significant weather information to the observation. When on duty, the observer is completely responsible for the observation, even though the automated weather observing system may generate and transmit the report. At facilities where augmentation is not available full time, the facility may be classified as automated during the non-augmented periods Manual Station. An activity where certified weather observers are responsible for observing, evaluating and preparing the observation. At these facilities, various degrees of automated sensors and/or other automated equipment may be available; however, the observer is asolely responsible for the observation Towered Station. Any activity with an air traffic control (ATC) tower having ATC personnel reporting tower visibility. 2.3 Navy METOC Service Standards for Observing. The term METOC Service Standards refers to four levels of detail in weather observations and personnel manning at sites where there is an ASOS. Individual stations may operate at times with manning different from their assigned standard. These levels of service are currently applicable to U.S. Navy and civilian activities only Level D - completely automated service in which the ASOS observation constitutes the entire observation, i.e., no additional weather information is added by a human observer. Airfields with a level D service are usually auxiliary fields or practice runways. Service Level D fields may provide data only locally or may also be configured to transmit the observation via network to nearby manned stations Level C - consists of the ASOS and a human observer, who is present during peak traffic hours and adds information to the automated observation. This is referred to as "augmentation. The augmented observation includes, thunderstorms, tornadoes, 2-2

23 freezing precipitation, hail, virga, volcanic ash, tower visibility and any element operationally significant to flight operations. Additional visibility increments of 1/8, 1/16 and 0 and if observed, the following elements will be added to the observation: sector visibility, variable sky condition, cloud layers above 12,000 feet and cloud types, snow depth, lightning, widespread dust, sand and other obscurations, and volcanic eruptions. Service Level C also includes "backup" of ASOS elements in the event of an ASOS malfunction or an unrepresentative ASOS report. In the backup mode, the observer inserts the correct or missing value for the automated ASOS elements. During hours that the field is closed, the field reverts to stand-alone ASOS or Service Level D as described above Level B - consists of the ASOS and a human observer, who is present during all field operating hours and adds all the augmented data listed in Service Level C to the automated observation. These fields have higher than average bad weather operations for visibility, thunderstorms and/or freezing/frozen precipitation, are remote or have significant training functions. Service Level B also includes "backup" of ASOS elements in the event of an ASOS malfunction or an unrepresentative ASOS report. In the backup mode, the observer inserts the correct or missing value for the automated ASOS elements. During hours that the field is closed, the field reverts to stand-alone ASOS or Service Level D as described above Level A - consists of the ASOS and a human observer who is usually present continuously (24/7). This level of service is for fields that generally have 24/7 operations, high profile missions, and/or higher than average bad weather operations for visibility, thunderstorms and/or freezing/frozen precipitation, or are remote or have significant training functions. The observer augments the ASOS observation the same as a Service Level C station Determination of the Level of Service. In order to determine which fields would receive a particular service level of weather support, fields are ranked according to their scores in three areas: (1) occurrence of bad weather weighted by traffic counts; (2) distance to the nearest suitable alternate airport; and (3) critical airport characteristics. These criteria produce a score which determines the airport s level of service. The bad weather factor is calculated by taking into consideration the percentage of times that the airport is impacted three elements: low visibility, thunderstorms, and freezing precipitation. This percentage is then multiplied by the total number of operations at the airport. The greater distance to the nearest suitable alternate, the higher the number for this factor. The critical airport characteristics score is based upon a combination of assigned Tower Level (representing the complexity of the air traffic control mission), the average traffic count (number of take-offs and landings), number of flight weather briefs per month, and the number of weather warnings issued per month. The scores from the three areas described above were added together and each field was assigned a composite score and ranked accordingly. The overall ranking determined the airport s METOC Service Standard Level. This standard shall be reevaluated annually. 2-3

24 2.4 Requirement for Observations. Observing stations with ASOS take/transmit observations 24/7. ASOS stations shall augment the ASOS observations with a human observer at least one hour prior to the commencement of operations. Stations without ASOS shall ensure a manual observations is taken at least one hour prior to the commencement of operations. 2.5 Observations During "Not Augmented Hours". Prior to securing, the observer shall log off the ASOS and ensure the system is in the automatic mode. Aviation Weather Hubs/Centers shall ensure that an ASOS observation is continuously available for remotely supported stations either by a network or the ASOS reporting system. If the automatic observation is not available, observers may be called in to manually take and transmit observations. 2.6 Observing Locations. The observing location is defined as the point or points at which the various elements are observed. In cases where all the measurements are taken at the same point, an observation will be regarded as having a single location. In cases where the various sensors are located to obtain acceptable exposure, the observation location will be regarded as varying with the individual elements in an observation. Normally, multiple observing points are confined to an area within about 2 miles of the station. Weather reports from manual stations may also contain information on phenomena occurring at other than the location of the observation. Regardless of observing location or the locations of the sensors, there shall be only one observation disseminated for a field. At most fields the observation location is defined as follows: Primary with ASOS. For clouds, prevailing visibility, present weather, wind, temperature, and dew point, the observing location is normally the location of the ASOS sensor suite. If there are two ASOS units, it will be the ASOS located near the active runway. For augmented data, it will be near the observing station where there is an unobstructed view, preferably across from the center point of the main runway Primary for Manual and Backup. For all elements except pressure, near the observing station where there is an unobstructed view, preferably across from the center point of the main runway in a grassy area. For pressure, it is inside the weather station. 2.7 Control Tower Observations and Weather Station Actions. FAA orders require certified control tower personnel to make tower prevailing and sector visibility observations when the prevailing visibilities at the usual point of observation, or at the tower level, are less than 4 miles. NAVMETOCCOMINST G provides procedures to certify control tower personnel to take visibility observations Required ATC personnel Actions: Notify the weather unit when they observe tower prevailing visibility to decrease to less than, or increase to equal or exceed 4 miles (6000 meters) Report all changes to tower visibility of one or more reportable values to the weather unit when the prevailing visibility at the tower or the surface is less than 4 miles. 2-4

25 Use the lower of either the tower or surface visibility as the reported visibility for aircraft operations as required by the FAA Required Weather Activity Personnel Actions: Notify the tower as soon as possible, whenever the prevailing visibility at the weather activity s observation point decreases to less than, or increases to equal or exceed 4 miles (6000 meters) Re-evaluate surface prevailing or sector visibility, as soon as practicable, upon initial receipt of a differing control tower value, and upon receipt of subsequent reportable changes at the control tower level Use control tower values of prevailing or sector visibility as a guide in determining the surface visibility when the view of portions of the horizon is obstructed by buildings, aircraft, etc. The presence of a surface-based obscuration, uniformly distributed to heights above the level of the tower, is sufficient reason to consider the weather unit's prevailing visibility to be the same as the control tower level Report the lower visibility value as prevailing and report the higher visibility in the remarks when the tower visibility is less than 4 miles and differs from the visibility at the surface point of observation. 2.8 Types of Surface Weather Observations and Codes. Surface weather observations are taken and encoded for three primary purposes. Aviation support generally requires continuous weather observations and uses FM 15 METAR and SPECI code forms for distribution. Numerical model support is generally provided via synoptic observations and code FM 12. Climatology support is achieved through the archive and processing of METAR, SPECI, and synoptic observations at the National Climatic Data Center in Asheville, NC. Although this manual addresses general observation procedures it s emphasis is on aviation support and only covers the METAR and SPECI codes METAR. This is the primary code format used to satisfy requirements for reporting hourly surface meteorological data to support aviation operations. A METAR observation may be prepared by an automated weather observing system (with or without augmentation) or by certified weather observers. A complete METAR contains the type of report, station identifier, date/time of observation, and whether the report is automated (AUTO) or corrected (COR). Weather phenomena reported in the METAR include wind, visibility, present weather, sky condition, temperature, dew point, and altimeter setting (collectively referred to as "the body of the report"). In addition, significant information elaborating on data reported in the body of the report, or coded and plain language data not included in the body of the report, may be appended in a section referred to as "remarks." The METAR is the scheduled observation. At manual stations, it is taken between 45 and 59 minutes past the hour. METAR observations are scheduled on the hour at ASOS sites. 2-5

26 2.8.2 SPECI. An unscheduled observation or special observation is taken when there is a significant change in the observation since the previous METAR observation or if an aircraft mishap has occurred. If it is time for a METAR to be issued, and SPECI observation criteria are met, the observation will remain designated as a METAR. SPECI criteria are applicable only to stations that have the capability of evaluating the event. Special observations shall be taken and transmitted according to the criteria provided in Table Synoptic Observations. Synoptic observation reports are designed primarily for use in numerical weather analysis and prediction. These observations are recorded at 0000, 0600, 1200, and 1800 UTC and may be reported at intermediate times of 0300, 0900, 1500, and 2100 UTC. Any station designated to provide synoptic reports shall enter the code on the lines following the recorded METAR for the same time, disregarding column headings if using paper forms. Federal Meteorological Handbook No. 2, Synoptic Code provides instructions pertaining to synoptic weather reports and codes. 2-6

27 CRITERIA FOR SPECI TABLE 2-1 ELEMENT CRITERIA THRESHOLDS A U T O M A T E Wind Shift Wind Direction changes by 45 Deg or more in less than 15 minutes and the speed is sustained at 10 knots or more throughout the shift. A U G M E N M A N U A L T E D D X X X Visibility Weather and Precipitation Surface visibility as reported in the body of the report decreases to less than or if below increases to equal or exceed: Tornado 3 Miles (4800 meters) X X X 2 Miles (3200 meters) X X X 1 Mile (1600 meters) X X X ½ mile or published field landing minima as listed in Dept of X X X Defense Flight Information Publications (DoD FLIP) Is Observed X X Disappears from sight or ends X X Thunderstorm Hail Freezing Rain or Drizzle Ice Pellets Begins (a special is not required to X report the beginning of a new X X thunderstorm if one is currently reported.) Ends X X X Begins X X Ends X X Begins X X X Ends X X X Changes intensity X X X Begin X X End X X Changes intensity X X 2-7

28 ELEMENT CRITERIA THRESHOLDS A U T O M A T E D Sky and Ceiling Volcanic Eruption Runway Conditions Aircraft Mishap Miscellaneous Change in Level of Service Cloud Layer CRITERIA FOR SPECI TABLE 2-1 cont Ceiling (rounded to reportable values) forms or dissipates below, decreases to less than, or if below, increases to equal or exceed. For runway conditions other than dry Upon occurrence at the station or notified of an aircraft emergency, unless there has been an intervening METAR or SPECI Any other meteorological condition considered operationally significant or designated by higher authority A SPECI will be taken within 15 minutes after the observer returns to duty following a break in observing coverage or augmentation unless a record observation is filed during that 15 minute period A layer of clouds or obscuring phenomena is present below 1,000 feet and no layer aloft was reported below 1,000 feet in the preceding METAR or SPECI A U G M E N T E D M A N U A L X X X 3000 Feet X X X 1500 Feet X X X 1000 Feet X X X 500 Feet X X X 200 Feet or all published landing minima (including circling) applicable to the airfield, as listed in the Department of Defense Flight Information Publications (DOD FLIPs) X X X Published airfields takeoff minima X X X When first noted X X Upon receipt from ATC personnel X X ACFT MSHP shall be placed in remarks; report not transmitted.. X X X X X X 2-8

29 2.9 Time Procedures. The accuracy of time in the reports is critical in aviation safety investigations. One clock will be designated the observing station standard, and a routine procedure established to assure its accuracy once a day at a minimum. The clock shall be within +/- 1 minute of the Master Clock at the U.S. Naval Observatory Actual Date and Time of Observation. For METAR reports, it is the actual date and time the last element (usually pressure) of the report that was observed or evaluated. For SPECI reports, it is the actual date and time the criterion for a SPECI was observed or sensed. The actual time for METAR observations shall be no earlier than H+55 and no later than H+59. This is the filing time of the observation Standard Time of Observation. The cardinal hour to which a record observation applies. For example, 1055Z is the 11th cardinal hour Time Checks. Perform a daily time check on the ASOS at 0800 local using the Naval Observatory WEB site or National Institute of Standards and Technology radio station W W V. Adjust as necessary Order of Reporting. Elements having the greatest rate of change are evaluated last. When conditions are relatively unchanging, evaluate the outdoor elements first and then evaluate any elements accessed indoors with pressure being evaluated last. Individual elements entered in a report shall, as closely as possible, reflect conditions existing at the actual time of report. Elements entered shall have been observed or evaluated within 15 minutes of the actual time of the report. Wind gusts shall be reported if observed within 10 minutes of the actual time of report Rounding Figures. Except where otherwise designated in this manual, if the fractional part of a positive number to be dropped is equal to or greater than one-half, the preceding digit shall be increased by one. If the fractional part of a negative number to be dropped is greater than one-half, the preceding digit shall be decreased by one. In all other cases, the preceding digit shall remain unchanged; e.g., 1.5 becomes 2, -1.5 becomes -1, 1.3 becomes 1, and -2.6 becomes Night Vision Adjustment. Personnel shall allow sufficient time for their eyes to become adjusted to the ambient light conditions before beginning to evaluate weather elements Certifications and Standards. In order to ensure validity and reliability of the Department of the Navy and the Federal Government s meteorological data, the use of standards, certifications and quality control must be an inherent program elements. Without these elements, the credibility of the any forecast and the nation s climatological database would be inconsistent and lack credibility Surface Weather Observer Certification. Prior to assuming full responsibility for taking or augmenting any type of surface weather observation or any part thereof, personnel shall be certified as required by FMH-1. Navy personnel shall be certified in accordance with NAVMETOCCOMINST K. Marine Corps personnel shall be qualified in accordance with MCO P (T & R Manual). 2-9

30 Tower Visibility Observer Certification. Air Traffic Control (ATC) personnel shall be task certified to evaluate visibility for both prevailing and sector visibility observations from the control tower. This training and certification is available on a CD Equipment Certifications. All instruments used for official observations shall meet the minimum standards for accuracy, range and resolutions as prescribed in FMH Automated Surface Observing System (ASOS) Certification. OFCM in the FMH- 1 requires federal agencies to attest that their weather stations adhere to standards applicable to all stations involved in the taking of surface weather observations used for aviation purposes. NAVAIR 00-80T-114 (NATOPS Air Traffic Control Manual) establishes naval policy that identifies the In-Service Engineering Activity (ISEA) as the authority for determining certification criteria for fielded equipment at naval shore Air Traffic Control facilities. Stations shall ensure that installed ASOS systems meet established certification criteria, and provide readings that are generally representative of the field conditions and are not influenced by vegetation, buildings, etc Algorithms Certification. The algorithms used in Navy and USMC ASOS must conform to the latest algorithms prescribed in the Federal Standard Algorithms for Automated Weather Observing Systems used for Aviation Purposes Documentation and Records. Observations are official records and as such must be documented and maintained either electronically or on paper Manual Retention and Archive. All manual observations shall be recorded on CNMOC 3140/12. After entry of all required data for the day or for the period of time that the ASOS is inoperative, the form is saved until the end of the month. A the end of the month, stations shall make a copy of the form for in-house retention. The original surface weather report forms along with a transmittal form, NAVMETOCCOM Form DF.shall be mailed by the fifth working day of the following month to: Officer in Charge FLENUMMETOC DET ASHEVILLE 151 Patton Avenue Asheville, NC Stations shall retain the copies of surface weather report forms for 1 year. Mobile units shall ensure all recorded observational data has been mailed upon completion of deployment. Records of classified observations must follow classified transmission requirements Electronic Archive and Records. ASOS has the capability to maintain and transmit data automatically including end of day and end of month summaries. Stations that implement this capability do not have a requirement to maintain paper copies for one year; however, if manual observations have to be taken due to ASOS failure, the paper copy of those observations must be retained as describe in the previous paragraph. Although stations may choose to maintain copies for their own use, the

31 day storage on the ASOS unit and the permanent automatic storage at the National Climatic Data Center are sufficient to achieve this purpose Station Information File. Each station shall prepare and submit NAVMETOCCOM Form DF, "Station Information File" (SIF), not later than 25 January annually, or at any time the instrumentation or its location is changed. The report is to reflect current station instrumentation as of 1 January of the year submitted. Instructions regarding the preparation of the SIF form and the form itself are provided on-line at the Fleet Numerical METOC Detachment s web site: The SIF form shall be sent electronically to FNMOD Asheville via . FNMOD Asheville does not need to have a signed SIF; the submitting and approving blocks still are required to be filled out and will be used instead of a signature. The accuracy of environmental studies derived from environmental observations recorded from instruments or sensors at a site depends upon correct documentation describing instruments and sensing elements, exposure, location, height above the ground, terrain features surrounding the station and other pertinent remarks regarding sensor performance Sensors and Instruments. NAVMETOCCOMINST () and the MSKC WEB Site contain information on allowances, installation, relocation, configuration, and support for sensors and instruments. Instruments and sensor suites vary greatly depending on the mission and location of the station; however, observations will only be taken using instruments that have been approved by COMNAVMETOCCOM or HQMC ASL-37 to take official weather observations Primary Sensors. Stations shall normally use the approved and centrally purchased automatic sensor suites as their primary means of observation. Although there are many low cost automatic instruments available commercially, most do not have the required accuracy or calibration programs, nor durability necessary to support aviation and shall not be used for the official observation. Sensor Standards are contained in FMH ASOS. Where installed, ASOS shall be the official observation suite of sensors at permanent or garrison stations. ASOS is the joint sensor suite used by FAA, DoD and DoC, and has continuous improvement and validation programs associated with it AWOS and other official automated sensors. Approved semi-portable sensor suites such as the Automated Weather Observing System incorporated into NITES IV and used by the Navy Mobile Environmental Teams and the Marine Corps METOC Support Teams meet the same standards for sensor accuracy as the ASOS. These sensor suites may or may not be tied into a computer containing the algorithms used by ASOS; however, the same rules on sensor observation and observation augmentation used for ASOS apply to this primary sensor suite Required Backup Instruments. All stations taking manual or augmented ASOS observations are required to maintain this backup instruments as specified in NAVMETOCCOMINST L for the phenomena specified: 2-11

32 Handheld anemometer (AMES RVM 96B ) or PMQ-3 for wind speed and direction Four inch plastic rain gauge (NOVALYNX ) for precipitation measurement Aneroid Barometer (ML448) for pressure measurements Psychrometer (either electric or manual) or hygrothermometer (such as Kestral, etc.) instrument for temperature and dew point measurements Calibration and Maintenance. Any weather observation is only as accurate as the sensors or instruments used to gather the data. Automatic sensors accuracy is primarily maintained via electronic calibration and preventive maintenance. Manual or mechanical instruments rely more on visual inspection or on instrument comparison. In either case, the station METOC personnel shall ensure that sensors and equipment are properly maintained and calibrated. When there is reason to believe that the accuracy or validity of sensors or instruments is questionable, personnel shall discontinue use until necessary corrective maintenance or calibration has been accomplished ASOS and AWOS Calibration, Maintenance and Sensor Representativeness. Navy and Marine Corps activities shall perform required user/operator maintenance in accordance with equipment manuals. Activities shall also ensure host/supporting activities Ground Electronic Maintenance Departments (GEMD) personnel provide maintenance and calibration to ASOS, in accordance with(iaw) S-100 Site Technical Manual. Calibration shall be performed at least annually and after any major maintenance. With the exception of the Aneroid Barometer, manual instruments do not usually have the accuracy of the ASOS sensors. However, station personnel shall visually check the ASOS sensors and compare with manual instruments to ascertain that the sensors are generally representative of field conditions and that no external structures, vegetation or other conditions are interfering with their readings Aneroid Barometer. As required by the FMH-1, the barometer shall be calibrated yearly in accordance with NAVAIR 17-35MTL-1, Metrology Requirements List (METRL) Other Manual Instruments. Other manual instruments should be periodically checked for physical damage and compared as specified in their associated technical manuals Observing Aids for Visibility. Activities shall post photographs, diagrams, lists, or other positive means of identifying lights or objects used as observing aids so that these aids can be accessed quickly and easily. Separate lists or charts should be used for daytime and nighttime markers Visibility Check Point Diagrams. Permanent stations shall submit a work order to have the local Civil Engineering (CE) agency survey objects to be used as markers. These objects shall be placed on a circular diagram illustrating their distance and 2-12

33 direction from the point of observation. Units deployed to other land areas should use available tools, such as Military Grid Reference System (MGRS) maps, laser range finder equipment, and Global Positioning System (GPS) devices to determine directions and distances to objects and to create tactical visibility diagram Selecting Markers. The most suitable daytime markers are prominent dark or nearly dark colored objects (such as buildings, chimneys, hills, or trees) observed in silhouette against a light-colored background, preferably the horizon sky. When using an object located in front of a terrestrial background, use caution when the object is located closer to the point of observation than it is to the terrestrial background. The most desirable night-visibility markers are unfocused lights of moderate intensity (about 25-candle power). The red or green runway course lights and TV or radio tower obstruction lights may be used. Do not use focused lights such as airway beacons due to their intensity; however, their brilliance may serve as an aid in estimating whether the visibility is greater or less than the distance to the light source Panoramic Photo. Stations shall also have a panoramic photo taken from the point of observation and showing as many of the visibility markers as possible. Representative visibility marker photos should be of high quality (color/digital) taken on a predominantly cloud and obscuration free (clear) day, and be representative of the current state of the airfield/site Periodic Review of Visibility Markers. Activities shall establish a process to review the Visibility Check Point Diagrams and photo at least annually and update if needed Control Tower Visibility Aids. ATC regulations require control towers to maintain a visibility checkpoint chart or list of visibility markers posted in the tower. Upon request, units will provide whatever assistance is necessary to help prepare a chart or markers of suitable objects for determining tower visibility Observation Quality Control. All stations shall stress the importance of quality control. The quality control performed at stations prior to dissemination is most important since once the observation is transmitted it is impossible to know that corrections have been brought to the attention of potential users. This can be especially significant for military operations in or near operational weather minimum situations. A pre-dissemination check should consist of recalculating computed data, verifying syntax of reports and comparing data to previous observations. Post-dissemination quality control should consist of checking the transmitted observation for errors that were generated during dissemination and for repetitious errors made by observers. Generally personnel should not be assigned to correct errors for the sole purpose of submitting an error free report to the Fleet Numerical METOC Detachment, Asheville. ASOS has several integrated quality control procedures built into the software. These include self-diagnostics as well as logic algorithms of the sensors. Still, the observer shall check all data before transmission, especially the subjective elements (see paragraph 3.5.2). 2-13

34 2.18 Dissemination. Weather observations must be disseminated to the operational users to be of any use; this can be as simple as verbally conveying the information to ATC and other base personnel or automatic electronic distribution via the World Wide Web Local. The transmission or delivery of the weather observation to offices on the same station or nearby supporting activity, especially the local ATC activity. This dissemination is normally done via internal computer systems, tele-writer, or telephone and shall normally be accomplished before other dissemination Long-line or Distant, Network. This transmission ensures that all other METOC offices and other activities receive the weather report and provides support for the National Airspace and International systems. This is accomplished via major networks and computer systems; it ensures aviators at other locations are aware of the conditions at any given reporting station Corrections to Previously Disseminated Reports. Corrections shall be transmitted as soon as possible; however, if erroneous data has been superseded by a later observation, it shall not be transmitted. Corrections transmitted shall consist of the entire corrected report. The original date and time of the observation shall be used in the corrected report. Refer to corrections paragraph Delayed Reports. When transmission of an observation is delayed until the next regularly scheduled observation is required, only the latest observation shall be transmitted. On the record of observations, the remark FIBI (Filed but impractical to transmit) will be entered in the remarks for the observation(s) not transmitted. When a SPECI observation is delayed, and another special is required, the later special shall be transmitted only when the overall change between the last transmitted observation and the current report satisfies the criteria for a special. Again, if the special observation is not transmitted, the remark FIBI will be entered. 2-14

35 CHAPTER 3 ASOS AUTOMATED OBSERVATIONS 3.1 Why Automate Weather Observations? Modern technology can now provide a near real-time measurement of the atmosphere. Doppler weather radar routinely updates its data sets every 5 to 6 minutes. Weather satellites provide new information every 15 to 30 minutes. Yet the routine surface observation taken by weather observers at airports across the nation were for years mostly a single hourly observation. When the weather is quickly changing, the human observer sent only a few extra observations highlighting those changes. Only by automating the surface observation could the timeliness of the surface weather information be integrated more effectively with the products of other systems. Automation removed the burden for humans to create observations at small airfields and at part time operations. The trained meteorological personnel were then better used to assist the forecast and warning. ASOS is the primary surface weather observing system used in the United States and at selected Naval overseas sites. There are approximately 1000 ASOS in the United States, most owned by the FAA and the NWS; the Navy and Marine Corps has approximate ASOS User s Guide and the ASOS Ready Reference Guide. The ASOS User s Guide is a joint NOAA, DoD, FAA and U.S. Navy document providing detailed instruction on the sensors, algorithms and use of ASOS. The National Weather Service s ASOS Ready Reference Guide provides detailed instruction on setup, user inputs, and operation of the system. All activities having an ASOS shall maintain the latest versions of these publications and use them along with this instruction as a constant reference for the station s observation program. Besides the required office paper copy, the documents can be found on the Web at the NWS site: ASOS Description. The ASOS consists of three main components and a video tower display. The first two components are at all ASOS locations; the third component is found only at airports where observers augment or provide backup support. The components are: a Sensor Group or groups, consisting of individual weather sensors and a Data Collection Package (DCP), an Acquisition Control Unit (ACU) usually located inside the office, and an Operator Interface Device (OID). The tower display is called a Video Display Unit (VDU). ASOS senses the elements and then processes the raw data through algorithms to provide a variety of observations. These observations or products are: 1-minute observations, 5-minute observations, METARS, SPECI, daily summary, and monthly summary. These observations may be distributed only locally within the activity, or the ASOS may be configured to transmit the observation to a wide area network (WAN) ASOS Sensor Group. (Fig 3-1) The ASOS sensors continuously sample and measure the ambient environment, derive raw sensor data and make it available to the collocated DCP. The raw sensor data include temperature, wind, precipitation, visibility extinction coefficients, ceilometer cloud hits, freezing precipitation resonant frequencies and other sensor data. These data are processed as preliminary input into the observation algorithms. 3-1

36 The ASOS consists of the following basic complement of sensors: Ceilometer, Cloud Height Indicator [CHI] Sensor Visibility Sensor Precipitation Identification (PI) Sensor Freezing Rain (ZR) Sensor * Lightning Sensor * Pressure Sensors (Located in the ACU) Ambient/Dew Point Temperature Sensor Anemometer (wind direction and speed sensor) Precipitation Accumulation Sensor (Heated Tipping Bucket [HTB] Gauge) * These sensors will be installed at selected Navy/USMC sites during the next ASOS upgrade. Figure 3-1 ASOS Sensor Group 3-2

37 3.3.2 Sensor Location. Sensors are sited in accordance with guidance stated in the Federal Standards for Siting Meteorological Sensors at Airports (FCM-S4-1987), published by the Office of the Federal Coordinator for Meteorological Services and Supporting Research (OFCM). At virtually all locations, the pressure sensors are located indoors within the ACU. The field complement of ASOS sensors is typically located near the Touchdown Zone (TDZ) of the primary designated instrument runway. If the TDZ site was found unacceptable, the Center Field (CF) location is the most likely alternate location. The field sensor array is referred to as the ASOS Combined Sensor Group. A DCP is located with the sensor group. It continually gathers and processes raw data from the adjacent sensors (e.g., voltages, extinction coefficients, data counts) and conditions these data before transmission to the ACU. Data conditioning may include such processes as sampling, formatting, and scaling Acquisition Control Unit. The ACU, which is the central processing unit for the ASOS, is usually located inside a climate controlled structure, such as an observing office or control tower building. It consists of the communication devices, computer, pressure sensors, and power supplies in an equipment cabinet. The ACU performs final processing, formatting, quality control, storage and retrieval of the data, and makes ASOS data available to users through various outlets. An ASOS ACU is shown in figure 3-2. Figure 3-2 ASOS Acquisition Control Unit (ACU) Operator Interface Device (OID). The OID is an interactive device that allows the operator to review, edit, generate, and monitor the processing of weather data including the METAR and SPECI observations. The OID consists of a display terminal and keyboard. The OID also enables the operator to access and invoke the primary functions of ASOS. The OID is an input/output device used for direct communication with the system s Central Processing Unit (CPU) housed inside the ACU. There are four user levels available to the operator when signing onto the system via the OID: observer (OBS), air traffic control specialist (ATC), electronics technician (TEC), and system manager (SYS). Each user level requires a site-specific password assigned by the system manager for access to certain OID functions. The system uses the password to determine the type of user (i.e., OBS, ATC, TEC, or SYS) signing onto the 3-3

38 system and the OID functions available to that user. In addition, another user level is available, the unsigned user (UNS), which does not require signing onto the system with a password. The unsigned user can review system data via the OID. Basic instructions on OID actions for augmented observations are given in Chapter 4. Detailed instructions on operator use of the OID are contained in the ASOS Ready Reference. Configuration procedures for the OID are contain in Appendix III of the ASOS Ready Reference Video Display Unit (VDU). This unit is a monitor that provides a simplified display of the ASOS data in the tower. It provides the current official observation, the oneminute observation and some additional data. It consists of a monitor only and does not have the keyboard or displays for entering data or controlling the ASOS. Most Navy towers also have a Visual Information Display System (VIDS) which provides ATC personnel with a variety of data; one piece of which is the current official observation. 3.4 Available ASOS Displays. A variety of data outputs are available from the OID. They include One-Minute Observation data (the basic or home display), the coded Aviation Routine Weather Reports (METAR) and Aviation Selected Special Weather Reports (SPECI), an Auxiliary Data Display (relative humidity, sea level pressure, station pressure, density altitude, pressure altitude and magnetic wind), and Maintenance Data (raw sensor data, system diagnostics, and system status). Various summary data outputs are also available METAR Elements. The ASOS will automatically report the following weather elements in the METAR: Wind - Direction (tens of degrees - true),- Speed (knots), and Character (gusts) Visibility up to and including 10 statute miles Basic present weather information: Rain, Snow, etc. and Intensity Obstructions: Fog, Mist, Haze, etc. Sky Condition: cloud height and amount (CLR, FEW, SCT, BKN, OVC) up to 12,000 feet above ground level Ambient temperature, dew point temperature (degrees Celsius) Pressure - altimeter setting in inches of mercury (Hg), and sea-level pressure (SLP) in hectopascals (hpa). Limited Plain Language Remarks (depending on service level) include: wind shift, beginning and ending of precipitation, SLP, 3, 6, 24-hour precipitation amount, hourly temperature and dew point, 6-hour maximum and minimum temperatures, 3-hour pressure tendency, various sensor status indicators, and maintenance check indicator ($) Auxiliary Data. These data are available and updated every minute as are the basic METAR elements. The auxiliary data are generally derived from other processed sensor data by the weather reporting algorithms, and, therefore, cannot be edited directly, but may be altered indirectly by editing of the component parameters through the OID. 3-4

39 3.4.3 Maintenance Data. These data are intended primarily for the electronics maintenance personnel and provide indications as to the operation of the various sensors and units Summary Data. These data are used primarily for climatology and record keeping purposes and may be transmitted automatically to the National Climatic Data Center. 3.5 The ASOS Observation Process. While the automated system and the human may differ in their methods of data collection and interpretation, both produce an observation quite similar in form and content. The weather elements observed by ASOS are categorized as either objective or subjective elements. Details on the mechanics and physics of the ASOS sensors and additional details on the algorithms beyond those given here are contained in the ASOS Users Guide The Objective Elements. These elements are classified as objective because they are simpler, are directly measured by one sensor, and are easier to automate than other elements. They can usually be represented by simple digits. These elements are ambient and dew point temperature, wind, pressure, and precipitation accumulation The Subjective Elements. While automating most objective weather elements is straightforward, there are numerous complexities in automating subjective visual elements such as sky condition, visibility, and present weather. The major problem is how to objectively quantify subjective human judgment. With subjective elements, observers traditionally read instruments and watch weather conditions in their area at a fixed time to produce a snap-shot observation - a method called spatial averaging. To create a similar observation, automated systems repetitively sample conditions in a relatively small volume of air and then average these data over a set period, a technique called time averaging. Technological advances have made it possible for modern observations to progress from the periodic, subjective, spatial averaging methodology of the observer to an objective, nearly continuous, temporal averaging method of automated observing systems. Additionally, determination of some elements is a composition of raw data derived from multiple sensors and various calculations. The rules for observing sky condition (clouds), visibility, present weather, and obscurations were designed for observers and are still defined for subjective use; however, the FMH-1 now includes expanded rules for automated techniques ASOS Techniques. For the "objective" elements such as pressure, temperature, dew point, both the ASOS and the human observer use a fixed location and near fixed time reading technique. For wind, both use a short time-averaging technique. The quantitative differences between the human observer and the ASOS observation of these elements are negligible. For the "subjective" elements; such as sky condition, visibility, and present weather; the human observer uses a fixed time, spatial averaging technique to describe the visual elements, while the ASOS uses a fixed location, time averaging technique. Although this was a fundamental change, manual observing techniques and ASOS techniques yielded remarkably similar results within the limits of their respective capabilities. The differences in some cases were subtle, and need to be fully understood for optimum utilization of the information provided by the ASOS. 3-5

40 3.5.4 ASOS Algorithms. The ASOS uses computer algorithms (organized processing steps) that conform with the Federal Standard Algorithms for Automated Weather Observing Systems Used for Aviation Purposes, published by the Office of the Federal Coordinator for Meteorology (OFCM). Individual elements encoded into the ASOS observation, as closely as possible, reflect conditions existing at the actual time of observation. For those elements that the weather technician evaluates using spatial averaging techniques (e.g., sky cover and visibility), the ASOS substitutes time averaging of the sensor data. Therefore, in an ASOS observation, sky condition is an evaluation of sensor data gathered during the 30-minute period ending at the actual time of observation. All other elements evaluated are based on sensor data that is within 10 minutes or less of the actual time of observation. The presence of other phenomena is determined by algorithms that evaluated data from multiple sensors (fog sensed by temp/dew point and visibility sensors, snow sensed by temp and precip sensor). In the descriptions of the algorithms, if less than 75 percent of the maximum amount of data used in the computation of any parameter is available, the parameter is not reported. Unless otherwise noted, all midpoint fractional values are rounded down to the nearest appropriate value. All other values are rounded in accordance with normal rounding procedures. 3.6 Human vs. ASOS METAR. Unlike human observers, ASOS provides a continuous weather watch and therefore is more likely to timely report significant changes in weather. Where available, its ability to automatically transmit METAR and SPECI ensure that the latest observation is available to the entire National Aviation networks. In the automatic mode, no observer is signed on to the ASOS at the OID and the ASOS will automatically transmit the METAR and any required SPECI to the outside networks. The system automatically generates SPECI observations with appropriate remarks for squalls, wind shifts, predefined visibility conditions, predefined sky conditions for ceilings and layers, beginning and ending of precipitation, thunderstorms - if equipped with lightning sensor, and freezing rain occurrences/changes in intensity - if equipped with freezing rain sensor. The unattended ASOS METAR and the attended ASOS (Augmented) METAR look very much alike. For example, under the same circumstances, the ASOS would report: KNGU Z AUTO 11006KT 5SM HZ SCT080 15/12 A3013 RMK AO2 SLP123 The observer who is editing the ASOS might report: KNGU Z 11006KT 5SM HZ SCT080 BKN140 15/12 A3013 RMK AO2 VSBY SE 2 SLP123 Notice that both METAR observations contain the station ID, observation type, time, wind, visibility, obstruction, sky condition, ambient temperature, dew point temperature, altimeter setting, and sea-level pressure (in remarks). The automated observation indicates the station type as AUTO which signifies an unattended observation (i.e., Observer not logged onto ASOS for back-up or augmentation), and identifies the system as one capable of reporting present weather (AO2). The second observation indicates that the Observer logged onto the system (AUTO is missing). The Observer augmented the sky condition for clouds above 12,000 feet and added information in the Remarks section about sector visibility that ASOS cannot provide. The ASOS Users Guide provides details and specific examples for each of the weather elements. Automated observing concepts are also discussed for greater clarity and understanding. 3-6

41 3.7 Quality Control of ASOS Observations. The automation of surface observations reduces or if in automatic mode eliminates direct human involvement in acquiring (sensing, collecting), processing (assessing, formatting, documenting), and disseminating (transmission, display, broadcast) of surface observations. Even though the ASOS is highly automated and reliable, effective quality control (QC) or monitoring of ASOS products is critical for ensuring the high level of trust in the automated output. There are up to three cascading levels of quality control for ASOS. Each level focuses on a different temporal and spatial scale Level 1. This QC is performed on-site, in real-time before an observations is transmitted. Built into each ASOS are automated self-diagnostics and QC algorithms. These QC algorithms operate on raw sensor data; they prevent questionable data from being included in the One-Minute-Observation (OMO) or the transmitted METAR/SPECI. The complete list of data quality checks is contained in Appendix I of the ASOS Ready Reference Guide. An example of one of these checks is the temperature must vary by more than.1 Degree over 120 minutes, and must not change more than 10 Degrees over 1 minute. When the ASOS detects either a qualifying degree of system degradation, component failure, or data error, the relevant data are excluded from report processing and a Maintenance Check Indicator ($) is appended to the METAR/SPECI report. If sufficient raw data are not available for report processing, the element is not included in the one-minute observations (OMO) or METAR/SPECI report. The $ is used to indicate that maintenance may be needed but does not necessarily mean that the data are erroneous. Onsite observers and in some cases ATC personnel often augment and/or backup the ASOS METAR/SPECI data. At these sites, the observers provide an immediate data check and often catch problems before the observation is transmitted Level 2. This check is performed at a Central Forecasting Site for a designated forecasting area. The check is usually accomplished within the hour after the scheduled observation transmission time. Navy and Marine Corps forecasting personnel routinely monitor and assess the availability and meteorological quality of long-line transmitted METAR and SPECI observations from all ASOS locations in their areas. If data are suspect, the forecasters investigate the problem, inform the points of contact (either the on-site observer or maintenance technician) at the activity experiencing the problem and coordinates corrective action to ensure that a quality observation is provided Level 3. This quality monitoring is performed at the National Weather Service s ASOS Operations and Monitoring Center (AOMC). This center monitors all US ASOS observations. If they detect an ASOS problem, they can first check out the system remotely via direct phone connection to the ASOS, and if the issue is software may be able to solve the problem. If not, the AOMC calls the responsible Navy or Marine Corps Aviation Hub, the METOC Systems Knowledge Center, and the responsible GEMD to affect repairs. 3-7

42 3.8 Ambient and Dew Point Temperature Observation Ambient and Dew Point Temperature Sensors. (Figure 3-3). The ambient and dew point sensors uses a platinum wire Resistive Temperature Device (RTD) to measure ambient temperature and a chilled mirror to determine dew point temperature. The RTD operates on the principle that electric resistance in a wire varies with temperature. This RTD is located in the stream of aspirated air entering the sensing unit and assumes the ambient air temperature. To determine dew point temperature, a mirror is cooled by a thermoelectric or Peltier cooler until dew or frost begins to condense on the mirror surface. The body of the mirror contains a platinum wire RTD, similar to that used for ambient temperature. This RTD assumes the mirror s temperature, which is held at the dew point temperature. When this condition occurs, the mirror s surface is in vapor pressure equilibrium with the surrounding air (i.e., has reached the saturation vapor pressure). The temperature required to maintain this equilibrium is, by definition, the dew point temperature. Optical techniques are used to detect the presence of surface condensation. Figure 3-3 Ambient and Dew Point Temperature Sensor Due to the high maintenance associated with the dew point portion of this sensor, it is being replaced by a new solid state dew point sensor. The new sensor for dewpoint measurement in the ASOS is based on warmed HUMICAP thin film polymer sensor technology. The measurement with capacitive thin film polymer sensor is itself very reliable and has a very low failure rate and is not susceptible to pollutions. Figure 3-4 New ASOS dry HUMICAP Dew Point Sensor Temperature and Dew Point Primary Data and Algorithm. Both ambient temperature and dew point temperature are considered conservative elements (i.e., continuous in space, and slowly and smoothly changing in time). Based on this characteristic, time-averaging over a short period is the preferred method of measurement. The ASOS hygrothermometer continually measures the ambient temperature and dew point temperature and provides sample values approximately six times per minute. Processing algorithms in the hygrothermometer use these samples to determine a 1-minute average temperature and dew point valid for a 60-second period ending at M+00. These data are passed to the ACU for further processing. Once each minute the ACU calculates the 5-minute average ambient temperature and dew point temperature from the 1-minute average observations (provided at least 4 valid 1-minute 3-8

43 averages are available). These 5-minute averages are rounded to the nearest degree Fahrenheit, converted to the nearest 0.1 degree Celsius, and reported once each minute as the 5-minute average ambient and dew point temperatures. The ACU performs a number of data quality tests on the ambient and dew point temperatures, including upper and lower limit checks, a rate of change check, and a cross comparison. The current 1-minute ambient and dew point temperature are compared against these limits. If the ASOS has not recorded a valid 5-minute average ambient or dew point temperature within the past 15-minutes, no ambient or dew point temperature is reported and the sensor failure notation is entered into the ASOS system maintenance log (SYSLOG). This 15-minute hold-off allows the daily 15-minute calibration heat cycle to occur without adverse affect. The current 1-minute average ambient temperature and the 5-minute average ambient temperature are updated once each minute and stored in memory for 12 hours. These stored data are used in further computation Other Temperature and Dew Point Data Once each minute, ASOS uses the running 5-minute average ambient temperature to update the hourly maximum and minimum ambient temperatures. At the end of the hour (H+59), the cumulative maximum and minimum ambient temperatures for the hour and the minute(s) they occurred are stored in memory for 24 hours At synoptic hour (00, 06, 12, and 18 UTC) reporting times, the current 6-hour maximum and minimum ambient temperatures are computed from the hourly maximum and minimum ambient temperatures in tenths of degrees Celsius, and included as remarks (1s n T x T x T x for maximum temperature, and 2s n T n T n T n for minimum temperature) in the current synoptic hourly METAR The current 12-hour average ambient temperature is also computed once each minute from the current and previous 12-hour reported 5-minute average temperature (this value is used for calculating current sea-level pressure) Once each hour (at the hourly METAR report time) the current hourly ambient temperature and dew point temperature are reported in the METAR Remarks section, to the nearest tenth of a degree Celsius, in the form Ts n T T T s n T d T d T d Once each minute, ASOS computes the highest and lowest ambient temperatures, so far for the current calendar day, in tenths of degrees Celsius. The calendar day maximum and minimum ambient temperatures are reported in the midnight, LST hourly METAR remarks (4s n T x T x T x s n T n T n T n ), and are stored in memory for 31 days as part of the Daily Summary Product. The midnight LST hourly METAR is usually transmitted before 23:59 LST. Therefore, there may occasionally be a discrepancy between the maximum and minimum temperatures reported in the 4 group and the daily maximum and minimum temperatures reported in the ASOS Daily Summary Message (DSM) which runs from 00:00 to 23:59 LST Once each day (at 23:59 LST), the highest and lowest ambient temperatures for the current month, along with the date(s) of occurrence, are computed and stored in 3-9

44 memory until the end of the following month. On the first day of the following month, ASOS outputs the Monthly Maximum Temperature and date(s) of occurrence, plus the Monthly Minimum Temperature and date(s) of occurrence Relative humidity is calculated using the 5-minute average ambient (dry-bulb) temperature and dew point temperature. The 5-minute average temperature is also used to process other algorithms, specifically sky condition (obscuration determination), present weather (freezing rain determination, and snow - rain discrimination), obscurations (freezing fog, fog, mist, haze discrimination), and pressure (sea-level pressure, pressure altitude, and density altitude calculation) ASOS Temperature/Dew Point Strengths and Limitations. Substantial improvements have been achieved by ASOS in measuring temperature and dew point by increasing the sensor aspiration and siting the sensors away from buildings and heat islands. Furthermore, continuous monitoring, self-diagnostics, and application of quality control algorithms of the ASOS ensure that any ambient temperature/dew point temperature degradation trend is reported before sensor performance falls below performance standards. At times, however, the reported dew point temperature may become stuck at around zero degrees Celsius. At other times, it may not be representative because of excessive mirror contamination due to dust or other atmospheric aerosols. An aggressive preventative maintenance program must be conducted that includes periodic cleaning of the mirror surface. 3.9 Wind Observations Wind Sensors (Figure 3-5). The rotating cup anemometer and the simple wind vane are the principal indicators of wind speed and direction. The basic method of observing wind direction and speed is to take a fixed point, time averaged measurement. ASOS continuously and objectively measures wind direction and speed once every second, far more frequently, consistently, and accurately than an observer could. Every ASOS processes data identically, which provides site-to site consistency unknown in past records. The ASOS wind sensor employs an electro-optical method to determine wind speed and convert it to appropriate electro-magnetic signals. Figure 3-5 ASOS Wind Sensor The sensor s starting threshold for response to wind direction and wind speed is 2 knots. Winds measured at 2 knots or less are reported as calm. Typical ASOS wind sensor heights are 33 feet or 27 feet, depending on local site-specific restrictions or requirements Wind Algorithm and Primary Data. ASOS reports a 2-minute average of 5- second average wind directions once a minute (i.e., 24 samples each minute). These five-second averages are the fundamental units used to compute reportable wind 3-10

45 values and are, in effect, the ASOS equivalent to the manual instantaneous wind observation. The current 2-minute average wind direction is updated on the OID screen display once every minute and included in the transmitted METAR/SPECI messages. The direction from which the wind is blowing is reported to the nearest 10-degree increment (e.g., 274 degrees is reported as 270 degrees). Wind direction is reported relative to true north in the METAR/SPECI message, in the daily/monthly summaries, and on all video displays. Wind direction is reported relative to magnetic North as well as True on the VIDS data display screen in the tower. A 2-minute average of wind speed is updated once every 5 seconds and is reported once every minute, displayed on the OID and included in the METAR/SPECI message Other Wind Data. Wind Character information is added to the METAR after the Wind Direction and Speed data when the variability in the steady state wind exceeds threshold criteria. Wind Character components include Wind Gusts and Variable Wind. Although Wind Squalls are reported as a Present Weather phenomenon in METAR, they are also discussed here for comparison and contrast. ASOS obtains the wind character by comparing the difference between the average wind with the instantaneous wind speed observed over a specified time interval. When this difference exceeds a prescribed value, the appropriate additional wind information is included in the observation. Besides wind character data, the ASOS can determine various time special wind data peak wind and wind shifts Gusts. Like manual wind observation techniques, the ASOS algorithm relies on a 10-minute observation period to determine gusts, but uses it in a different way. Once every 5 seconds, the ASOS computes the greatest 5-second average wind speed (and corresponding direction) during the past minute, and once each minute stores this information in memory for 12 hours. Once every 5 seconds the ASOS computes the current 2-minute average wind speed and compares it with the greatest 5-second average wind speed during the past minute. If the current 2-minute average wind speed is equal to or greater than 9 knots and the greatest 5-second average wind speed (during the past minute) exceeds the current 2-minute average speed by 5-knots or more, then the greatest 5-second average speed observed during the past minute is stored in memory as a gust for 10 minutes. Once every 5 seconds, the ASOS compares the highest gust stored in memory for the past 10 minutes with the current 2- minute average wind speed. If the difference between the two is 3 knots or more, the current reported wind speed is greater than 2 knots, and the highest gust exceeds the minimum 5-second wind speed in the past 10 minutes by 10 knots or more, then the highest gust stored in memory is designated as the reportable gust. This value is appended to the current wind direction and speed reported in the OMO and the METAR/SPECI reports. The minimum gust speed reported by ASOS is 14 knots. Wind speeds from 3 knots and 11 knots may be reported with gusts to 14 knots. For example, a 2-minute average wind of 240 degrees at 10 knots with gusts to 20 knots is reported as: 24010G20KT Variable Wind. The ASOS algorithm uses the same definition for determining a variable wind as the manual technique but uses different methods for reporting it. In both cases, a variable wind is reported when the wind direction varies by 60 degrees or more during the 2-minute evaluation period before the observation. If the 2-minute wind 3-11

46 speed is 6 knots or less, then a variable wind direction indicator (VRB) is included in the basic wind group; if the 2-minute wind speed is greater than 6 knots, then a variable wind direction group is appended to the basic wind group in the body of the METAR/SPECI report. The ASOS algorithm compares the range of 5-second average wind directions during the past 2 minutes (24 samples). In either case, if the current 2- minute average wind speed is 6 knots or less, the wind direction and speed is reported as VRBff, where ff is the current 2-minute average wind speed in knots. For example, a variable wind at 3 knots is encoded as VRB03. If the current 2-minute average wind speed is greater than 6 knots, then the current wind direction and speed are placed in the body of the report and followed by a variability indicator in the form d n d n d n Vd x d x d x, where d n d n d n is the minimum, and d x d x d x is the maximum variable wind direction during the past 2-minutes. For example, a current 2-minute wind of 270 degrees at 10 knots that varies from 240 degrees to 300 degrees is coded as: KT 240V Squalls. ASOS algorithm computes a potential squall value once every 5 seconds. If the current 2-minute wind speed (measured every five seconds) is greater than or equal to 22 knots and exceeds the 2-minute average wind speed computed two minutes ago by 16-knots or more, then the highest 5-second average wind speed during the last 2-minutes is stored in memory as a squall for 10 minutes. Only the current squall or non-squall default value is stored in memory. The stored squall value is reported as SQ in the present weather field if the current 2-minute average wind speed is greater than 2 knots, and if the squall value exceeds the current 2-minute average wind speed by more than 3 knots. According to this algorithm, a squall may continue to be reported by the ASOS for up to 10 minutes after the squall is written to memory, provided the above minimum wind-speed and squall reporting conditions are met. The minimum wind speed - squall combination reported by ASOS is a wind speed of 3 knots, with a squall of 22 knots Peak Wind. The Peak Wind, by definition, is the highest instantaneous wind speed observed or recorded since the last scheduled hourly observation (METAR). In ASOS, the Peak Wind is determined from the highest observed 5-second average wind speed that exceeds 25 knots since the last generated METAR. The Peak Wind direction, speed and time of occurrence are reported in METAR remarks as: PK WND dddff(f)/(hh)mm, where ddd = direction (true) in tens of degrees, ff(f) = wind speed in knots, and (hh)mm = (hour) minutes past current hour of most recent occurrence of the reported peak wind Wind Shift. The ASOS algorithm uses the same definition of a wind shift as described in the FMH-1: A wind shift is indicated by a change in wind direction of 45 degrees or more in less than 15 minutes with sustained winds of 10 knots or more throughout the wind shift. The ASOS determines a wind shift by first making sure that minimum wind speed and direction change criteria are met. These checks are made to ensure that light and variable winds are not erroneously reported as a wind shift. The wind speed criterion requires that all 2-minute average wind speeds computed each minute over the past 15 minutes are greater than 9 knots. If this criterion is met, then the current 10-minute average wind direction derived from ten one-minute-observations is compared to a similar 10-minute average wind direction from 15 minutes ago. If the 3-12

47 wind directions differ by 45 degrees or more, then a wind shift is encoded. The wind shift remark generated by the ASOS in the METAR report is: WSHFT hhmm, where hhmm is the Universal Coordinated Time (UTC) of when the shift began (15 minutes ago). Once the wind shift remark is reported, it will continue to be included in all subsequent reports (including long-line dissemination of ASOS generated SPECI) through the next scheduled hourly METAR ASOS Wind Strengths and Limitations. The major strength of the ASOS in reporting winds is the consistency of measurements. While Observers rely on perception and human judgment to interpret wind instruments, automated systems rely on digital second-by-second measurements that are processed identically from time-totime and place-to-place. One limitation in the automated observation is a lag in reporting wind shifts (in METAR remarks). The wind shift algorithm cannot rely on external clues used by the observer (like thunder or snow showers) for early collateral assurance of a wind shift - frontal passage occurrence. It must therefore wait the full 15-minutes required in the definition of wind shift before outputting a remark. Although highly unlikely, a wind shift and variable wind remark may both be generated and included on the same METAR under conditions of light and variable winds which just barely meet the wind shift reporting criteria, or when a FROPA has occurred in the preceding minutes Pressure Observations Pressure Sensor. The ASOS pressure measurement instrument consists of redundant digital pressure transducers, which use capacitive sensors, one side of which is permanently evacuated to a vacuum to make it a barometric pressure sensor. Advanced microcomputer electronics and sophisticated firmware provide reliable performance. The barometers are located on a tray at the bottom of the ACU and are exposed to the ambient air pressure. In cases when the ACU is installed in pressurized buildings, this exposure is through a port connected to an outside static pressure vent Pressure Algorithm and Primary Data. A sophisticated algorithm routinely computes and updates the pressure report for each pressure sensor once a minute from readings obtained every 10 seconds from each sensor. If one or more of the 6 pressure readings obtained from each sensor in the past minute is missing, then the 1- minute pressure value for that sensor is marked as missing and the sensor is logged as inoperative. The current 1-minute pressure values from each sensor are then compared against each other and absolute differences computed. The lowest 1-minute sensor pressure value obtained from a pair of sensors, whose pressure difference is 0.04 inch or less, is the designated ASOS pressure to be reported at the end of the minute. A sensor whose 1-minute sensor value differs by more than 0.04 inch from another operational sensor is automatically logged as inoperative, and the sensor pressure value set to missing. This will cause a maintenance check indicator to be appended to all subsequent ASOS METAR/SPECI reports until the sensor is returned to an operational status. Once a pressure sensor is logged as inoperative, it can only be returned to an operational status by a maintenance technician. If one of the sensors (at two-sensor locations), or if two of the sensors (at three-sensor locations) are logged 3-13

48 as inoperative, then the designated ASOS pressure elements are all omitted in the METAR/SPECI reports Other Pressure Data. The lowest 1-minute sensor pressure value obtained from a pair of sensors pressure value is then used to compute altimeter setting, sea level pressure, density altitude, pressure altitude and pressure rising/falling rapidly using standard equations and input from the temperature data. These are straight mathematical calculations ASOS Pressure Strengths and Limitations. The pressure sensors are the most reliable and accurate sensors in ASOS. The only limitation (if one can call it that) is that pressure remarks will be reported more often in ASOS METAR messages than in manual METAR messages simply because of the continuous weather watch which ASOS provides Precipitation Accumulation Observations Precipitation Gauge. (Figure 3-6) The Heated Tipping Bucket (HTB) gauge yields accurate measurements of most precipitation events except extremely heavy rainfall. The ASOS HTB shown consists of 6 main components. Figure 3-6 Heated Tipping Bucket Wind shield. A wind shield that surrounds the HTB and protects it against blowing snow falling into the HTB collector funnel. The wind shield is installed on the ASOS HTB in climates where the snowfall is > 20% of the annual precipitation accumulation inch Diameter Collector Funnel. This funnel provided the correct magnification of the rainfall for the size of the tipping bucket Pivoting Dual Chamber Tipping Bucket. This bucket tips when one chamber is filled with liquid precipitation that is equivalent to.01 inch over the surface area of the funnel. As it empties its contents into a drain pan, it causes the other chamber to be expose to the precipitation gathered by the collector funnel Electronic Switch. This switch senses each tip of the bucket Drain pan and a drain tube Heating Elements. To prevent freeze-up during cold weather there are two heating elements. One heating element is wrapped around the underside of the 3-14

49 collector funnel, and the other around the drain tube. Each heater is thermostatically controlled to maintain a temperature of 40 F The Precipitation Accumulation Algorithm and Primary Data. The precipitation accumulation algorithm obtains precipitation accumulation data from the HTB precipitation gauge once each minute. These data are valid for discrete 60-second periods ending at M+00. The ASOS algorithm corrects the tipping bucket measurement, particularly during periods of high rainfall rates. Each minute the measured rainfall is adjusted using the following equation: C = A (1 +.60A) In this equation, C = the calculated rainfall amount and A = the measured amount from the tipping bucket. All calculations are performed internally using floating point until rounded for final output each minute. The basic METAR Prrrr remark reports rrrr which is the liquid equivalent of all precipitation (in hundredths of an inch) that has occurred since computation of the Prrrr remark for the last scheduled hourly METAR message. The rrrr is the sum of all 1-minute precipitation accumulations calculated during this period. If any of the required constituent 1-minute precipitation accumulation calculations are missing, the remark is omitted and PNO is appended to the remarks section to indicate that precipitation accumulation information is not available. If no precipitation has occurred since the last scheduled hourly METAR, the Prrrr remark is not reported. If only a trace of precipitation has occurred, the Prrrr remark will be reported as P0000. Furthermore, when the precipitation accumulation sensor is inoperative, a PNO remark will be appended to the METAR Other Precipitation Amount Data. When available, the output data are used each minute by the algorithm to compile a variety of other cumulative precipitation remarks/messages METAR 3 and 6-hourly report, 6RRRR precipitation accumulation remark: RRRR is the amount of precipitation, in hundredths of an inch, which has accumulated in the past 3- or 6-hours. Three-hourly amounts are reported in the 03, 09, 15, and 21 UTC METAR reports. Six-hourly amounts are reported in the 00, 06, 12, and 18 UTC METAR reports. If any of the constituent hourly Prrrr remarks are missing, the 6RRRR remark is encoded as 6//// METAR 24-Hour 7RRRR Precipitation Accumulation Remark. RRRR in this message is the amount of liquid equivalent in hundredths of an inch accumulated over the last 24-hours. This remark is reported with the 1200 UTC METAR provided there has been at least 0.01 inch of precipitation in the past 24-hours (i.e., since the last 1200 UTC METAR). The precipitation accumulation reported in the 7RRRR remark is compiled from the hourly precipitation computations. If any hourly amount is missing during the 24-hour period, the 7RRRR remark is encoded as 7//// Daily and Monthly Cumulative Precipitation Totals. These precipitation totals are summed each minute and included along with other data in the Daily Summary and Monthly Summary Products/messages ASOS Precipitation Accumulation Strengths and Limitations. A significant problem occurs during high rainfall rate events when the tipping bucket cannot keep up 3-15

50 with the water flow and under-reports the accumulation. The ASOS software corrects for the HTB bias to under report precipitation accumulation during most heavy rainfall events (greater than 1.80 inch per hour); however, during extremely heavy rainfall events (greater than 10 inches per hour), the HTB may still under-report the total rainfall accumulation. During freezing conditions, the application of heat to melt snow and prevent gauge icing also induces some evaporation or sublimation, especially during light freezing rain or snow events at temperatures near 32 F. This results in a tendency to under-report freezing or frozen precipitation accumulation Sky Condition Observations. Sky condition observation by its very nature is a subjective process. The complete evaluation of sky condition involves two sensors, the cloud height sensor (CHI) and under certain circumstances the visibility sensor. Although the CHI used by ASOS employs a different process than a human observer, comparison testing over several years indicates the ASOS sensor derived observations produce nearly identical results to those of a human observer. More important it is continuously observing the sky so it is more likely to detect significant changes to the sky condition Cloud Height Sensor (Figure 3-7). The ASOS uses a laser beam ceilometer with a reporting range of 12,000 feet. This CHI is a vertically pointed laser transmitter and receiver. Its operation is similar to radar in that the time interval between pulse transmission and reflected reception from a cloud base is used to determine the cloud height. Sophisticated time-averaging algorithms in the ACU are also used to interpret cloud hit information from the CHI and determine cloud height and amount. The width of the laser beam is confined to a divergence of ± 2.5 milliradians (mrad) so that at 12,000 feet the beam s sample area is a circle with a diameter of 60 feet. The CHI reports will contain only opaque clouds. Moisture layers, or thin clouds detected by the CHI and considered too thin to be a cloud, will be reported as a restriction to vertical visibility or simply not reported. The reporting of vertical visibility is dependent on the thickness and density of the moisture layer. To correctly classify these signals received by the ceilometer, sensor software processes the data into three categories: no hit, cloud hit, and unknown hit. The signal signature of a cloud return or cloud hit is characterized by a rapid increase in backscatter when the beam passes from the clear air beneath the cloud into the moist conditions within the cloud. The cloud base hits (or returns) from each pulse are assigned to one of the foot vertical data bins within the 12,600 foot measurement range. This results in a vertical resolution of 50 feet. Those signals without sharp Figure 3-7 Ceilometer 3-16

51 returns are classified as unknown hits. These unknown hits are primarily caused by precipitation and fog that mask the base of the clouds. This broadened moisture field returns laser backscatter signals from various heights within the moisture field. Because these signals cannot be processed as a definite cloud return, they are processed as a vertical visibility (VV) Basic Sky Condition Algorithm. The sky condition algorithms are quite complicated; what follows is a simplified explanation. Every 30 seconds a sample is compiled from the CHI backscatter returns taken from the most recent two or three 12- second processing intervals completed within the 30 second period. Each 12-second interval processes more than 9,000 signals for backscatter returns. These data are processed to determine the height of the returns and whether the sample compiled from these returns is a cloud hit or an unknown hit. Every minute, ASOS processes the most recent 30 minutes of 30-second sample data; the last 10 minutes of data are processed twice (double weighted) to be more responsive to the latest changes in sky condition. This technique provides 80 samples with 40 in the first 20 minutes and 40 in the last 10 minutes Determining the Layers. The cloud signal hits for the latest 30 minutes are then rounded or binned to the nearest 100 feet for cloud heights between the surface and 5,000 feet; to the nearest 200 feet for heights between 5,000 and 10,000 feet; and to the nearest 500 feet for heights above 10,000 feet. Each minute, if more than five bin height values have been recorded (during the last 30 minutes), the cloud heights are clustered into layers using a least-square statistical procedure until there are only five bins remaining (each bin can have many hits in it). These bins, or clusters, are then ordered from lowest to highest height. Following this clustering, ASOS determines whether the clusters can be combined into meteorologically significant height groups. This second clustering is done so that very close layers are not reported (e.g., BKN030 OVC032). At the end of this combining process, all cluster heights between the surface and 5,000 feet are rounded to the nearest 100 feet. Above 5,000 feet, the algorithm rounds the cluster height values to the nearest reportable value (i.e. nearest 500 ft. up to 10,000 ft. and nearest 1,000 ft. above 10,000 ft.). These bins now are called layers and the algorithm will select up to three of these layers to be reported in the METAR/SPECI in accordance with cloud layer reporting priority as specified in FMH Determining the Amount. The amount of sky cover is determined by adding the total number of hits in each layer and computing the ratio of those hits to the total possible. If there is more than one layer, the hits in the first layer are added to the second (and third) to obtain overall coverage. For reporting purposes, the ASOS measured cloud amount for each layer is then converted to a statistical functional equivalent of a human observation. All cloud layer heights are reported Above Ground Level (AGL) with respect to field elevation. The cloud amounts reported by ASOS are Clear (CLR) below 12,000 feet, Few (FEW), Scattered (SCT), Broken (BKN), or Overcast (OVC). Table 3-1 shows the relationship between the ASOS measured cloud amount, the human equivalent, and the reported category. 3-17

52 ASOS MEASURED AMOUNT IN % OF SKY COVER HUMAN EQUIVALENT IN OKTAS ASOS REPORT 00 </= 05 0 CLR >05 to </= 25 >0 to 2/8 FEW >25 to </= 50 >2/8 to </= 4/8 SCT >50 to </= 87 >4/8 to < 8/8 BKN >87 to 100 8/8 OVC Table 3-1 ASOS Cloud Amount Report - Percent of Sky Obscurations. The sky condition algorithm also tests for total obscurations. Necessary conditions for reporting a totally obscured sky include a surface visibility of one mile or less and a high percentage of unknown hits at or below 2,000 feet AGL. When these conditions are met, ASOS processes and formats cloud return values classified as unknown hits into the sky condition report as VVaaa. VV is an obscuration; aaa represents the visible height in hundreds of feet an observer can see vertically into the obscuring phenomena Variable Ceiling. When a ceiling layer (BKN or OVC) is reported below 3,000 feet AGL, an algorithm tests for conditions requiring a variable ceiling remark. If these conditions exist, ASOS places the entry of CIG minvmax (where min, max is the height in hundreds of feet of the upper and lower limits of the variability) in the remarks section Special Report Generation. The algorithm also generates SPECI reports for all of the ceiling and low cloud criteria as specified in FMH-1 and set up by the station ASOS Sky Condition Strengths and Limitations. As a whole the ASOS provides more accurate height data than human observers. ASOS continuously monitors the sky condition and automatically creates and issues a SPECI report with a speed and responsiveness, and is not limited to partial observation times associated with a limited weather watch common at most human only observer stations. The ceilometer generally works equally well day and night and does not suffer from night adaptation as human observers who must wait for their eyes to adapt at night. Finally, ASOS reports more broken cloud layers and fewer overcast layers than an observer because the ceilometer does not suffer from the packing effect. The packing effect is a condition where the observer tends to overestimate the cloud coverage because clouds near the horizon appear to blend together or overlap. This effect is due to the curvature of the earth and parallax view of distant objects. On the other hand, the ASOS ceilometer is a vertically pointing laser and therefore measures only the height and infers the amount of cloud elements that pass over the sensor. The sensor does not measure or know what is happening along the horizon, nor does it report on clouds above 12,000 feet. Due to the inherent motion of the atmosphere however, a single CHI is usually sufficient to report accurate cloud conditions. A common concern is that the algorithm does not respond fast enough to changes in the sky condition. If a solid 3-18

53 cloud deck suddenly appears, the algorithm will report a FEW layer within 2 minutes and a BKN ceiling layer within 10 minutes. If the CHI becomes inoperative for three consecutive 30-second readings (1.5 minutes) or misses five or more readings in the most recent 30 minutes, it does not report sky condition. The algorithm requires a full 30-minutes of data from the CHI after restart before a valid sky condition report is again generated. Precipitation poses a double challenge. Because the laser can only look vertically, precipitation particles directly overhead will scatter the laser light, often leading to an increased number of unknown signal returns. The algorithms described above help distinguish a vertical obscuration caused by falling raindrops from the base of the cloud layer above it. Precipitation can also fall onto or collect on top of the instrument. To limit that problem, a blower is used to move air over the slanted glass cover housing of the laser lens. Occasionally, the laser CHI can see too many clouds. For example, a very sensitive laser will sometimes detect invisible moist layers before they coalesce to become visible clouds. Also, occasional sharp inversions, especially in very cold winter conditions, may trigger the report of clouds. There are times when the CHI may report fewer clouds than the observer. Studies have shown that up to 20 percent of the reports of FEW cloud events will be missed by using only one ceilometer. These events are usually widely scattered fair weather cumulus, which the ASOS may report as CLR. Finally, although not an accuracy problem, the CHI tends to generate more specials than a human observer does. This is because the continuous observation allows the CHI to detect far more SPECI ceiling criteria than a human observer would normally note. This is especially true when there are rapidly varying amounts of low clouds Visibility Observations. NWS scientists developed the concept of Sensor Equivalent Visibility (SEV). SEV was defined as any equivalent of human visibility derived from instrument measurements. This means that visibility is not measured directly but is inferred by measuring other physical characteristics and properties of the air. These properties include the transmittance (or conversely attenuation) of light due to absorption and scattering by atmospheric contaminants such as aerosols, course particulates, and hydrometeors. Studies have shown that human visibility is closely correlated with transmittance. A sensor can measure this atmospheric capacity and quantify it through sophisticated algorithms to represent visibility. This is SEV. Figure 3-8 Visibility Sensor Visibility Sensor (Figure 3-8). The ASOS visibility sensor measures the transmittance and determines SEV using a forward scatter principle in which light is transmitted twice a second in a cone-shaped beam over a range of angles. To optimally balance the detection efficiency and differentiation ability of the sensor under varying conditions, a nominal 45 degree horizontal incident angle is set between the projector beam and the detector 3-19

54 field-of-view within the sampling volume. Consequently, the projector beam does not directly impinge on the detector lens. Only that portion of the beam that is scattered forward by the intervening medium in the sampling volume is received by the detector; see Figure 3-8. A measurement sample is taken every 30 seconds. The projector and detector are protected with a lens hood and canted down to prevent snow blockage. The detector looks north to minimize sun glare, particularly from low sun angles near sunrise or sunset. A photocell on the visibility sensor turns on at dawn or off at dusk at a light level between 0.5 and 3 foot candles (deep twilight). This function determines whether the sensor uses the day or night equation to calculate visibility Visibility Algorithm and Primary Data. The ASOS visibility algorithm samples sensor data every minute, obtaining a 1-minute average extinction coefficient and day/night indication. The algorithm calculates 1-minute average visibility every minute and stores the value for 10 minutes. Finally, ASOS computes a running 10-minute harmonic mean once per minute from the stored data to provide the latest visibility. A harmonic mean is used in the final computation rather than an arithmetic mean because it is less sensitive to outliers and more responsive to rapidly decreasing visibility conditions and will generally yield a lower value than the arithmetic mean. This result is preferable because it provides an earlier warning of deteriorating conditions. Conversely, when visibilities are rising rapidly, the harmonic mean will be slower than the arithmetic mean in reporting the improving visibility. This conservative bias is intended to provide an additional margin of aviation safety. The difference between the harmonic mean and the arithmetic mean can be seen when a fog bank moves in and suddenly drops the visibility from 10 miles to 1/4 mile. Initially there are 10, 1-minute values of visibility equal to 10 miles and both the arithmetic and harmonic mean are reporting visibility as 10 miles. In the first minute, there are nine values of 10 miles and one value of 1/4 mile used to compute the current visibility. The arithmetic mean yields miles while the harmonic mean yields miles. In the second minute, there are now eight values of 10 miles and two values of 1/4 mile used to compute current visibility. In this case, the arithmetic mean is miles and the harmonic mean is miles. The value obtained from this computation is then rounded down to the nearest reportable visibility value. Visibility in METAR is reported in statute miles (SM). The reportable increments are: M1/4SM, (less than1/4sm), 1/4SM, 1/2SM, 3/4SM, 1SM, 1 1/4SM, 1 1/2SM, 1 3/4SM, 2SM, 2 1/2SM, 3SM, 4SM, 5SM, 6SM, 7SM, 8SM, 9SM and 10SM. Note that visibilities between zero and less than 1/4 mile are reported as M1/4SM. Measured visibilities exactly half way between reportable values are rounded down. Visibilities of 10 miles or greater are reported as 10SM. Eight oneminute data samples in the last 10 minutes are required to form a report. If less than 8 of the current 10, 1-minute visibility values are available in memory, the 1-minute visibility is not reported by ASOS Special Report Generation. The newest visibility value is checked against the visibility reported in the last METAR or SPECI report to determine if it has passed any specified visibility thresholds. If so, ASOS creates a SPECI report Variable Visibility. The algorithm also checks for variable visibility. If requirements for variable visibility are met, (i.e. varying between two reportable values) ASOS generates an appropriate automated remark. 3-20

55 ASOS Visibility Strengths and Limitations. One of the main advantages of the visibility sensor is consistency of observations. Under the same weather conditions, an ASOS visibility reported at one site would be identical to the visibility reported at another site because both ASOS s use identical sensors and algorithms. Variations introduced by human observers from such limitations as perspective, sun angle, day and night differences, light affects on the eye and poor locations are eliminated. The major disadvantage of an automated forward scatter sensor is the small sampling volume (0.75 cubic feet) which can give undue weight to small-scale differences in the atmosphere. Even broadening the sample by including all the sensor measurements over a 10 minute period (20 samples) may not provide enough data to create a visibility report that represents the entire airport area. For instance, if patches of fog (BCFG) or shallow fog (MIFG) form, or a fog bank moves over one end of the runway, ASOS may not see it if the sensor is not in the right location Present Weather and Obscuration Observations. The automated detection and reporting of present weather and obstructions requires whole new observing technologies and techniques. Present weather and obstructions arguably are the most complex and difficult elements to automate due to their variety of composition and appearance. The ASOS employs a variety of sensors to correctly report present weather and obscurations. The precipitation identification (PI) sensor discriminates between rain and snow. A new lightning sensor soon to be installed at selected Naval stations provides information on thunderstorms and a future freezing rain sensor will detect ice accretion caused by freezing precipitation. The visibility sensor and hygrothermometer provide data for algorithms that further refine the reports of present weather and infer the existence and type of obscuration. Figure 3-9 Precipitation Identification Sensor Precipitation Identification Sensor (Figure 3-9). The Precipitation Identification sensor (PI), better known as a Light Emitting Diode Weather Identifier (LEDWI), is capable of identifying and reporting rain, snow, unknown precipitation (UP) and freezing precipitation (only at locations with a freezing precipitation sensor). It also determines the intensity of the precipitation. The LEDWI contains a coherent light transmitter (i.e. there is a continuous relationship among the various phases of the light waves within the beam) and a photo diode receiver. The transmitter and receiver are mounted on a crossarm 10 feet above the ground or base of the platform. They are equipped with heated lens hoods, face directly at each other, are separated by a distance of 2 feet and are oriented in a north-south direction with the receiver looking north. The transmitter generates a coherent Infrared light beam, 50 millimeters in diameter, aimed directly at the receiver. The receiver lens is masked with a narrow 1 millimeter horizontal slit aperture through which the 3-21

56 transmitter light beam passes before it is focused by the lens and impinges on the photo diode. As a particle of rain or snow passes through the coherent light beam, the particle creates a shadow that modulates the light, which then passes through the receiver s horizontal slit aperture as a partially coherent (intermittently disrupted), colliminated (parallel to the slit) beam. The shadow varies depending on the size and speed of descent of the particle as it falls across the receiver. When many particles fall through the beam, a scintillation pattern is created. The fluctuating beam pattern is sensed by a photo diode and amplified, creating a jumble of frequencies containing information on the size and speed of the falling particles. A spectral analysis reveals how much energy or power is contained in the various frequency bands. For example, a predominance of power in low frequencies from 75 to 250 Hz indicates snow. When energy is predominantly in a band from 1000 to 4000 Hz, the precipitant is almost certainly rain. The LEDWI registers rain and snow mixed as a smearing of the spectral power, which is usually reported by ASOS as unknown precipitation (UP). This analysis is the basis of the discrimination algorithm, which differentiates rain from snow. When the precipitation is not mixed (i.e., pure rain or pure snow) the LEDWI can determine the intensity of the precipitation. The intensity is determined by the power of the signal return in the rain (1-4 khz) or snow ( Hz) portion of the power spectrum. The LEDWI report of snow intensity is well correlated with the rate of snow accumulation but is not directly related to the visibility reduction due to snow, which observers use to differentiate between light, moderate, and heavy snow. ASOS processing algorithms use the visibility sensor data to modify the LEDWI snow intensity report so that moderate snow is not reported when the visibility is greater than 1/2 mile and heavy snow is not reported when the visibility is greater than 1/4 SM Precipitation Identification Algorithm and Primary Data. Every minute, the PI algorithm requests the PI sensor data, stores the data in memory for 12 hours, and examines the latest 10 minutes of data stored in memory. If three or more samples are missing, the algorithm sets the sensor status to inoperative and reports missing for that minute. If the 10-minute memory buffer contains less than two sensor samples of precipitation, the precipitation report terminates. If, however, two or more samples in the latest 10 minutes indicate precipitation, the algorithm then determines the type and intensity to report. In general, to report anything other than Unknown Precipitation (UP), two of the samples are required to be the same type. If there is a tie between two types of precipitation (e.g., two rain samples and two snow samples) snow is reported. This determination is based on the hierarchal scheme for reporting present weather: liquid, freezing, and frozen in ascending order. ASOS reports only one precipitation type at a time. For instance, if both freezing rain and snow are detected, snow is reported. Additional precipitation elements may be added to the report by observers. The PI algorithm performs a temperature logic check on the PI sensor output. If -SN, SN, or +SN is reported, and the temperature is > 38 F, then the PI sensor output is No Precipitation (NP). Once the precipitation type is determined from the last 10 minutes of data, then the 1-minute samples from the past 5 minutes are used to compute intensity. Precipitation intensity is determined from the highest common intensity derived from three or more samples. Possible intensities for a heavy precipitation are light, moderate and heavy; for moderate precipitation, possible common intensities are light and moderate. For example, if rain is the determined precipitation type, and there are three moderate rain and one light rain detected in the past 5 minutes, then ASOS 3-22

57 reports moderate rain in the METAR/SPECI report. Likewise, if snow is the determined type and there are one light, two moderate, and one heavy snow in the past 5 minutes, then moderate snow (highest common) is reported. If, on the other hand, there are less than three common intensities of the reported precipitation type, then ASOS reports the lightest intensity. For example, if rain is the determined type, and there is only one moderate rain and one heavy rain reported by the sensor, then the precipitation intensity is set to moderate rain (RA) in the ASOS METAR/SPECI report. If snow is the determined precipitation type and there are only one moderate and one heavy snow, then ASOS reports moderate snow. Under blowing snow conditions, particularly where snow is blown to a height of 10 feet or more, the PI sensor (LEDWI) can mistakenly interpret the scintillation from blowing snow rising up and/or settling through the sensor s IRED beam as rain, and occasionally as snow, depending on the vertical velocity of the snow particles. ASOS reevaluates all LEDWI reports of rain with an ambient temperature of 32 Deg F or less and using the blowing snow algorithm considers the LEDWI data and other ASOS sensors for concurrent blowing snow conditions. The algorithm evaluates sky condition and 15-minute average data for ambient temperature and wind. When all data are available, ASOS reports blowing snow when: Visibility is less than 7 statute miles Ambient temperature is 14 F or less Sky cover is less than overcast or the ceiling height is greater than 10,000 feet Wind speed is greater than 22 knots When this error is detected, ASOS replaces the false rain report with a correct blowing snow report in the ASOS METAR/SPECI present weather field. If these conditions are not met, ASOS reports UP Other Precipitation Data. The ASOS PI algorithm formats and reports precipitation beginning and ending remarks just as the observer does. The PI sensor output is further compared with the freezing rain sensor output (where available) to ultimately determine the precipitation reports and remarks (see description of the freezing rain algorithm, para , for further details) ASOS Precipitation Indicator Strengths and Limitations. Because of its continuous monitoring capability, the ASOS PI sensor often detects and reports the beginning and end of rain or snow before an observer. In addition, because of its sensitivity, the PI sensor can detect light precipitation even at times when it cannot distinguish between rain and snow (as the observer can). In these situations, the ASOS may report too many UPs. Because drizzle particles are small and fall slower than raindrops, their power spectrum is weak and smeared; therefore, drizzle is often not detected and is not reported. On occasion, however, it may elevate either the high or the low frequency channel of the PI sensor sufficiently to be reported as rain or snow. When light drizzle is falling, the PI sensor may, on occasion, interpret the scintillation pattern created by these suspended water droplets as light snow when the temperature is < 38 F. Product improvement efforts are aimed at reliably discriminating between drizzle, ice pellets (PL) and rain, which at times can have similar size and fall velocity. Consequently, ice pellets will force an output from the PI sensor of either UP, -RA, RA, or +RA (depending on intensity of the ice pellets). The ASOS processing algorithm will, 3-23

58 in turn, also report either -RA, RA, or +RA as output by the PI sensor. ASOS may report other types of frozen precipitation as rain or snow, depending on the scintillation pattern caused by their size and fall velocity. For example, snow grains are larger than drizzle and fall slower so they are sometimes reported as snow. Snow pellets also fall slower than raindrops and will therefore be reported as snow. Hailstones are larger than raindrops and fall faster, and therefore will be reported as rain. In mixed precipitation conditions, such as snow and ice pellets, the PI sensor likely interprets the ice pellets as rain. The snow, however, has a higher reporting precedence and thus snow will be reported by the processing algorithm. A mixture of snow grains and drizzle may also be reported as snow. In cases where snow grains are mixed with freezing drizzle, ASOS will report either snow or no precipitation. Sometimes the LEDWI may sense non-atmospheric phenomena. For example, insects flying in the receiver field of view can cause it to report precipitation. Spiders that run threads between the cross arms can trigger a false precipitation report when the wind moves the threads up and down. Even sun glint, particularly on a bright day when there are snow crystals (diamond dust) floating in the air, can trigger false precipitation indications. Strong winds during snow and blowing snow occasionally pack the receiver or projector heads of the LEDWI with snow. A total blockage of the lens will cause a loss of any detected signal creating an error condition. A partial blockage may contaminate the measurement and lead to inaccurate reports of precipitation type or intensity. The LEDWI has an adaptive baseline, which adjusts the power spectrum threshold to reduce reports of false precipitation. For instance, wind turbulence and thermals (e.g., the shimmer seen across an open field on a sunny day) can induce scintillation that is near the frequencies characteristic of snow. Therefore, ASOS sets the threshold for snow detection above the spectral power induced by turbulence. This adaptive baseline can pose a problem when snow increases so slowly that the baseline rises without snow being detected. When this occurs, only a sudden increase in snow may trigger the sensor to report snow. A similar condition may occur when the LEDWI is turned on (say after a power failure) and precipitation is falling. The initial adaptive baseline may be set much too high to detect precipitation correctly and will not be reset to a representative threshold until the precipitation ends. Rain detection is generally not a problem. Occasionally, if the rain is preceded by gradually increasing drizzle, the rain channel adaptive baseline threshold may rise to a point where light rain is not sensed. Generally, rain lighter than 0.01 inches per hour will not be detected. Figure 3-10 Single Site Lightning Sensor Single Site Lightning Sensor (Figure 3-10). Selected Navy and Marine Corps ASOS are currently being upgraded with a single site lightning sensor as a source for reporting a thunderstorm. The ASOS Lightning Sensor (ALS) is a singlepoint omni-directional system that requires two criteria before reporting a thunderstorm an optical flash and an electrical field change (radio signal), which occur within milliseconds of each other. The requirement for simultaneous optical and radio signals virtually eliminates the possibility of a false alarm from errant light sources. The sensor can detect cloud-to-ground and cloud-to-cloud strikes. All strikes are counted, but only the cloud 3-24

59 to-ground strikes are used to generate an estimate of the range. Cloud-to-ground strikes are grouped into three range bins: 0 to 5 miles, 5 to 10 miles, and 10 to 30 miles. Because the cloud-to-cloud detection is less efficient than cloud-to-ground detection, the ASOS considers cloud-to-cloud strikes to be within 5 miles. A cloud-to-ground strike is made up of one or more individual flashes. Within one flash, numerous discharges can occur; these individual discharges are called strokes. The sensor groups all strokes occurring within 1 second of each other into a single flash. The range of a cloud-toground strike is determined by the range of the closest stroke within a flash. The sensor automatically ages each lightning strike for 15 minutes. Because a thunderstorm is defined to be in progress for 15 minutes after the last lightning or thunder occurs, the sensor continues to report each strike in the appropriate bin for 15 minutes after it is first detected. A thunderstorm is determined to end when no strikes are detected within the last 15 minutes Single Site Lightning Sensor Algorithm. ASOS polls and processes data from the single site lightning sensor once a minute. To determine the starting and stopping times of a thunderstorm, ASOS examines data in the 0-5 and 5-10 mile cloud-to-ground range bins and in the cloud-to-cloud bin. For the single site lightning sensor, ASOS does not use data in the 10-to-30 mile range bin. A thunderstorm is declared to start when the sum of strikes in the three bins (0-5, 5-10, and cloud-to-cloud) is equal to, or greater than, two. To minimize the possibility of a false alarm, the ASOS algorithm requires two strikes to start a thunderstorm; false alarms are more likely with a single stroke starting algorithm. ASOS declares a thunderstorm to end when the last strike is aged out of the three bins and the sum of the strikes in the three bins falls to zero. There are two possible thunderstorm reports: TS when strikes are occurring within 5 miles, and VCTS when strikes are occurring outside 5 miles but within 10 miles. ASOS transmits a SPECI report at the start and end of any thunderstorm condition. To begin a TS report, the two-or-more strike condition must occur in one of the following formats: 0-5 bin only Cloud-to-cloud bin only 0-5 and 5-10 bins 0-5 and cloud-to-cloud bins Cloud-to-cloud and 5-10 bins 0-5, 5-10, and cloud-to-cloud bins A VCTS report is transmitted when the strikes occur only in the 5-10 mile bin. If a thunderstorm begins as a VCTS report, and a single strike is detected in either the 0-5 or the cloud-to-cloud bin, the report is changed to TS. Similarly, a TS report will be changed to VCTS when the only strikes detected are occurring in the 5-10 mile bin ASOS Lightning Sensor Strengths and Limitations. The ALS occasionally reports valid lightning within 10 miles when thunder cannot be heard by an observer, primarily because an observer s ability to hear thunder at most airports is restricted. Due to the two-part criterion (optical and radio) for identifying lightning strikes, the sensor has an inherently low false alarm rate. The 1997 field evaluation indicated a false alarm rate of (i.e., about 0.4 percent of ASOS thunderstorm minutes may be false). The ranging estimates provided by the sensor have significant uncertainty 3-25

60 and can contribute to reporting thunderstorms when cells are beyond 10 miles. A very strong lightning strike at miles can be erroneously placed in the 5-10 mile bin by the ASOS sensor. The cloud-to-cloud bin was retained in the ASOS algorithm because of the importance of cloud-to-cloud lightning in identifying thunderstorms. A significant number of thunderstorms contain primarily cloud-to-cloud lightning and if it were eliminated from the reporting algorithm, there would be a risk of failing to report thunderstorms Freezing Precipitation Sensor. (Figure 3-11) The Navy will be installing this sensor in the time frame at selected Navy and Marine Corps sites. The sensing device consists of a small cylindrical probe that is electrically stimulated to vibrate at its resonant frequency. A feedback coil is used to measure the vibration frequency, which is proportional to the mass of the probe. This magnetostrictive oscillator has the property of changing its magnetization due to a change in its (axial) dimension. In the ASOS sensor, the probe is driven at a natural resonant frequency of 40kHz. The axial vibration is of such low amplitude that it cannot be seen or felt. The probe is orientated vertically to provide optimal uniform exposure to freezing precipitation regardless of wind direction. This position also prevents birds from alighting. When ice freezes on the probe, the combined mass increases and the resonant vibration frequency decreases. There is a well-defined relationship between the measured frequency and the ice accretion on the probe. The freezing rain instrument is sensitive enough to measure accumulation rates as low as 0.01 of an inch per hour. The freezing precipitation sensor continuously monitors the resonant frequency of the vibrating probe, obtains a sample once a second, and once each minute averages the results to update the probe s current resonant frequency. When excessive freezing precipitation accumulates, (i.e., equal to or greater than 0.08 inch) the sensor goes into a heating cycle to melt the freezing precipitation from the probe and return it to the base resonating frequency. This process normally takes two to three minutes. During this time, the sensor status is set to deice and the output is not updated. Figure 3-11 Freezing Precipitation Sensor Freezing Rain Algorithm. Once each minute, the freezing precipitation algorithm accesses the current 5-minute average ambient temperature and the current frequency output from the freezing precipitation sensor and stores this data in memory for 12 hours. Data from the last 15 minutes are used to compute a current 1-minute freezing rain report. The report is FZRA if the following combined conditions are met: If the current ice accretion exceeds inch If the current ice accretion exceeds the minimum accretion since the sensor was last declared operational or during the past 15 minutes, whichever is less, by inch If the current five-minute ambient temperature is less than 37 o F 3-26

61 Otherwise, the freezing rain report is set to indicate a lack of freezing rain for the current minute. The freezing rain report for each minute is saved for 15 minutes. Finally, the freezing rain report is checked against the present weather report from the previous minute and special alert criteria for the beginning and ending of freezing precipitation, or changes in intensity, and if necessary, a SPECI will be generated. The intensity of the freezing rain is determined by the precipitation Identification Sensor. If three or more freezing precipitation sensor outputs in the last 15 minutes are missing, the sensor status is set to inoperative, and the freezing rain report is set to missing. Only the highest priority precipitation phenomena observed will be reported at any one time (i.e., ASOS does not report mixed or multiple precipitation types). For example, if the freezing rain sensor indicates -FZRA and the precipitation identifier reports UP, then the ASOS will report -FZRA. The previous minutes present weather reports are further examined to determine which present weather remarks need to be generated and appended to the METAR report. This includes precipitation beginning and ending times in minutes past the current hour (e.g., FZRAB05E21). Once freezing rain has been sensed and the ambient air temperature is 36 o F or below, it will be encoded in subsequent METAR reports for 15 minutes after it is no longer sensed ASOS Freezing Rain Strengths and Limitations. The major strength of the freezing rain sensor is its sensitivity and continuous monitoring, usually allowing it to detect freezing precipitation conditions before an observer, especially at night. Working in conjunction with other sensors, mixed precipitation events can be better defined. For example, under conditions of mixed precipitation, such as ice pellets (PL) and freezing rain (FZRA), the PI sensor will output UP, -RA, RA, or +RA (for the PL), but the FZRA sensor will output FZRA. The ASOS processing algorithms will interpret and correctly report this condition as -FZRA or FZRA. At temperatures near freezing, snow can become attached to the probe firmly enough to cause a frequency shift that could be misinterpreted as freezing rain. This problem of differentiating snow from freezing precipitation is overcome by checking the output of the PI sensor to determine if it is raining or snowing when the freezing rain sensor has a frequency shift. If the PI sensor indicates rain, then the report will be freezing rain. If the PI sensor indicates snow, then snow will be reported in the body of the METAR message. Most significantly, the current algorithm only reports freezing rain not freezing drizzle. Even if the freezing precipitation sensor senses icing, as long as the precipitation identifier sensor does not sense any precipitation (as would be the case in many drizzle situations) no freezing precipitation is reported Obscuration Algorithm. Obstructions are not directly measured by ASOS, but inferred from the measurement of visibility, temperature, dew point, and present weather (precipitation). ASOS reports only four obstructions: fog (FG), mist (BR), freezing fog (FZFG) and haze (HZ). Other obscurations, such as dust, smoke, and blowing sand are not automatically reported by ASOS, but may be augmented by the observer. The obscuration algorithm checks the reported visibility once each minute. When the surface visibility drops below 7 statute miles, the algorithm obtains the current dew point depression (DD) to distinguish between FG, BR, and HZ. When the DD is greater than 4 Deg F and no precipitation is reported by the PI and freezing precipitation sensors, then HZ is reported as the obscuration; however, when precipitation is reported HZ is not reported. If the DD is less than or equal to 4 Deg F, then FG or BR 3-27

62 will be reported. Visibility will then be used to further differentiate between FG and BR. If the visibility is less than 7 miles and down to 5/8ths of a mile, BR is reported. If the visibility is less than 5/8ths of a mile, FG is reported; however, if the ambient temperature is also below freezing, freezing fog (FZFG) is reported. When precipitation is reported, FG or BR may also be reported when the preceding conditions are met. In the event the DD is missing, (i.e., temperature and/or dew point is missing) the obscuration algorithm relies solely on visibility to discriminate between FG, BR and HZ. If the reported visibility is less than 7 miles and equal to or greater than 4 miles, HZ is reported. If the visibility is less than 4 miles and down to 5/8ths of a mile, then BR is reported as the obscuration. If visibility is less than 5/8ths of a mile, FG is reported. When present weather is also reported, FG or BR is appended to the present weather report. If reported visibility is equal to or greater than 4 miles but less than 7 miles, and no present weather is reported, then HZ is reported as the obscuration ASOS Obscuration Algorithm Strengths and Limitations. One obvious characteristic of the obscuration algorithm is that BR cannot be reported, even if it is the only obstruction to vision, when the DD is greater than 4 degrees F or when DD is missing and visibility is 4 miles or more. In these situations, if BR is actually occurring, then HZ is incorrectly reported. The ASOS, of course, cannot report obscuration remarks for distant phenomena such as FG BANK NE-SE. The ASOS, on the other hand, will provide more timely reporting of the formation and dissipation of obscuring phenomena due to its continuous monitoring capability Present Weather Algorithm Processing Matrix (Figure 3-13). This matrix should help simplify the explanation of the ASOS present weather algorithm processing. The algorithm assumes that all sensors are present and uses input from the Light Emitting Diode Weather Identifier (LEDWI), the frequency from the freezing rain sensor, the 10-minute harmonic average visibility, the dew point depression computed from the 5-minute average ambient and dew point temperatures, and the 5-minute average ambient temperature. Based on these parameters, ASOS will evaluate the data and provide an automated present weather entry (i.e., both precipitation and/or obscurations) to be included in the METAR/SPECI reports. 3-28

63 LEDWI Output Freezing Rain Indication Visibility (VIS) Dew Point Depression (DD) Temperature (T) ASOS PRESENT WX Algorithm Output P Yes or No NA NA T > 38F -RA P Yes NA NA T = 37F UP P Yes NA NA 0F < T < 36F -FZRA P No NA NA 0F < T< 38F UP P Yes or No NA NA T < 0F UP R Yes NA NA T > 36F RA R Yes NA NA 0F < T < 36F FZRA R No NA NA T > 33F RA R No NA NA 0F < T < 32F BLSN, UP, or No Entry R Yes or No NA NA T < 0F No Entry S Yes or No NA NA T > 38F No Entry S Yes or No NA NA T < 38F SN R, S, None Yes or No VIS >5/8SM < 7 SM DD < 4F Any T Value BR R, S, None Yes or No VIS < 5/8 SM DD < 4F T > 32F FG R, S, None Yes or No VIS < 5/8 SM DD < 4F T < 32F FZFG None Yes or No VIS < 7 SM DD > 4F NA HZ Legend P or UP - Light Unkown Precipitation None - No Precipitation Detected from LEDWI R or RA - Rain, (-RA is Light Rain) NA - Not Used in Algorithm S or SN - Snow FG - Fog FZRA - Freezing Rain, (-FZRA is Light Freezing Rain) FZFG - Freezing Fog BLSN - Blowing Snow HZ - Haze BR - Mist Table 3-2 Present Weather Algorithm Processing Matrix 3-29

64 3.16 ASOS Observations Dissemination. Besides the weather office OID display, ASOS provides output for the Video Information Distribution System (VIDS) Display at the Air Traffic Control areas including a METAR observation. Additionally there are two means to transmit the observation into networks. The observation may be manually or automatically transferred to another computer or weather display system/server, displayed on a web-page and transmitted to a central site via the Internet or the observation may also be transmitted directly by the ASOS. This can be an entirely automatic process or have human intervention whereby the observer checks and/or augments the observation. Either process allows the observation to be entered into a weather network so that all stations worldwide have access to the observation ASOS Data Archive and Recall. ASOS retains reports locally for various times depending on the frequency of the report. The 5-minute observations are retained and available for the past 12 hours. In addition, the observer can archive up to three 2 hour snapshots of 5 minute observations. The system maintains all METAR and SPECI observations generated for the past 31-days. It also contains all sign on/off actions. Instructions for recalling this data are contained in the ASOS Ready Reference Guide Data Not Provided by ASOS. The elements not currently sensed or reported by ASOS are: Tornado, Funnel Cloud, Waterspout Hail Ice Crystals Snow Grains, Ice Pellets, Snow Pellets Drizzle, Freezing Drizzle Volcanic Ash Blowing Obstructions (Sand, Dust, Spray) Smoke Snowfall (accumulation rate) and Snow Depth Hourly Snow Increase (SNINCR) remarks Liquid Equivalent of Frozen Precipitation Water Equivalent of snow on the ground Clouds above 12,000 feet Virga Partial Obscurations, surface based Types of Lightning Distant precipitation in mountainous areas, and distant clouds obscuring mountains Sector Visibility Significant Cloud Types 3-30

65 3.18 Planned Improvements. Besides the previously mentioned Naval upgrades of lightning sensor, freezing rain sensor and automatic transmit, there are several other planned improvements. Some stations would benefit from additional sensor suites at touchdown points; the Navy ASOS Intermediate Support Electronics Activity is investigating these possibilities. The National Weather Service has an extensive program to address many of the shortfalls listed in Paragraph Among the sensors and algorithms being developed and likely to be incorporated soon are: Drizzle & Freezing Drizzle Sensors/Algorithm An Ice Free Wind Sensor An improved Ceilometer An improved Temperature/Dew Point Sensor. An enhanced Precipitation Identifier 3-31

66 CHAPTER 4 - AUGMENTATION OF ASOS OBSERVATIONS This chapter prescribes procedures and practices applicable to the augmentation of automated surface observations at all Navy and Marine Corps stations. Augmentation is the process of adding data to an automated observation that is beyond the sensing capabilities of the system or correcting automated data based on personal observation of the elements. Although ASOS is used throughout this chapter, the same procedures apply for any automatic observation system. Approved semi-portable sensor suites such as the Automated Weather Observing System incorporated into NITES IV and used by the Navy Mobile Environmental Teams and the Marine Corps METOC Support Teams meet the same standards for sensor accuracy as the ASOS. However, these sensor suites may or may not be tied into a computer containing the algorithms used by ASOS. To perform augmentation duties, the observer shall maintain situational awareness of current weather conditions and the ASOS-generated observations. Navy/USMC METOC units will develop augmentation procedures and clearly defined duty priorities to direct duties that do not conflict with augmentation requirements. In all cases, the highest priority will be flight safety. 4.1 Augmentation Requirements When. ASOS observations will be augmented continuously at METOC service level A, B and C stations when routine flight operations are occurring Who. Only certified observers shall log on to the ASOS and modify or augment the observation. Once logged on, these certified observers are responsible for the completeness and accuracy of the weather observation. It is important that at watch turnover the off-going observer logs off and the on-coming observer logs on so as to maintain proper accountability for the observations What. The observer is responsible for providing additional information that covers a larger area, when operationally significant. Augmentation of automated observations includes both an accuracy check of all elements sensed by ASOS, and the observing and reporting of the phenomena listed in the following subsections. Automated weather observing systems are, by design, viewing a smaller area than a human observer. Details on complete manual observation of the phenomena are in chapters 6 through 12. Reporting procedures for the observation are contained in Chapter 13 - Coding and Dissemination. Procedures for the manual recording of the data are contained in Chapter 14 - Entries on the Surface weather Report Form CNMOC 3140/12. Except as specified in the following subsections, observing procedures for augmentation shall be the same as specified for the corresponding manual observation. Once an observation has been augmented, the observer shall ensure the validity of the entire observation by deleting or changing the data as required. 4.2 Specific Element Augmentations. When on duty, the observer at Navy and Marine Corps stations shall provide the additional data listed in the below subparagraphs as well as any other data deemed operationally significant. At other times, data normally reported by ASOS may be reported incorrectly or missed. This data 4-1

67 must also be added by the observer when on duty. Some of the augmented data is reported in the body of the observations while other data is reported as remarks Tornadic Activity. ASOS does not sense tornadic activity. The term tornadic activity shall include funnel clouds, tornadoes, and waterspouts. A funnel cloud, tornado, or waterspout is considered to begin at the time it is observed by the observer. A funnel cloud, tornado, or waterspout is considered to end at the time it disappears from sight. These phenomena shall be reported in a SPECI observation whenever they are observed or disappear from sight. The time, direction and distance from station, and direction of movement of the tornado should be reported. At ASOS sites, the event will continue to be reported automatically until the observer deletes the entry Thunderstorms. If the site is equipped with a lightning sensor, the ASOS creates a SPECI observation for a thunderstorm when lightning is sensed and provides the beginning and ending times of thunderstorms. It continues to keep the thunderstorm as present weather as long as it senses lightning. When on duty, the observer shall supplement the thunderstorm entry with proximity, direction and movement data and lightning type information if known. If no lightning sensor is present, the observer shall enter a thunderstorm as present weather whenever thunder is first heard or lightning is observed over the station when the local noise level is sufficient to prevent hearing thunder. The observer must also enter all the remarks data for a thunderstorm including beginning and ending times. A thunderstorm is considered to end 15 minutes after the last occurrence of any of these criteria. When added by the observer, a thunderstorm will continue to be reported until the observer ends the thunderstorm and deletes the entry Hail. ASOS does not sense hail. Hail begins at the time it is first observed and ends when it is no longer falling. Although reported as a precipitation, no intensity shall be assigned to hail, i.e., the observer shall not characterize hail as light, moderate, or heavy. An entry or deletion of a hail report shall be made in the remarks section. Hail size shall be reported, if known. The event will continue to be reported automatically until the observer deletes the entry Volcanic Ash. ASOS does not sense volcanic ash. The observer shall report volcanic ash whenever it is observed at the station. It will be reported in the body of the report as an obscuration whenever observed. At ASOS sites, the event will continue to be reported automatically until the observer deletes the entry. A SPECI observation is not required when volcanic ash is observed. No intensity is assigned to volcanic ash, i.e., the observer shall not characterize volcanic ash as light, moderate, or heavy. Remarks are optional, but if the volcanic eruption producing the volcanic ash is observed, it shall be entered in remarks and a SPECI observation shall be generated Virga. ASOS can not determine the presence of virga. Virga is defined as precipitation falling from clouds but not reaching the ground. The observer shall report virga in remarks when observed. Virga is not considered to be present weather or an obscuration. There is no standard contraction used for virga. VIRGA is spelled out in full. The event will continue to be reported automatically until the observer deletes the entry or until after the next hourly observation. The remark VIRGA will not be 4-2

68 automatically kept in remarks of the observation past the next hourly observation. If virga persists, it shall be re-entered as a remark. No SPECI is required when virga is observed. No intensity is assigned to virga, i.e., the observer shall not characterize virga as light, moderate, or heavy. The direction of the virga from the site is optional Freezing Rain and Freezing Drizzle. If freezing drizzle is occurring, or the ASOS does not have a freezing rain sensor and freezing rain is occurring, the observer shall report the occurrence of these precipitations as present weather and report the start, stop or change in intensity in the remarks section Ice Pellets. ASOS can not detect ice pellets. It will usually report them as rain. The observer shall report the occurrence of Ice Pellets as present weather and report the start, stop or change in intensity in the remarks section. Stations shall generate a SPECI when ice pellets begin, end, or change intensity Snow Pellets, Snow Grains, and Ice Crystals. ASOS does not sense these precipitations. The observer shall report the occurrence of these precipitations as present weather and the start and stop time in the remarks section Snow Depth Remarks. ASOS does not sense snow depth. Remarks such as snow increasing rapidly and snow depth shall be entered in the remarks section Lightning Not at the station. ASOS senses lightning generally only out to 10 miles when equipped with a lightning sensor. The observer shall report in remarks the direction of phenomena and the frequency of lightning Dust, Sand, Spray, and Smoke (obstructions not sensed by ASOS) shall be added to the obstructions and present weather if they reduce visibility to below 7 SM Cloud layers above 12,000 feet (height and amount) shall be reported Variable Sky Condition shall be reported in remarks Visibility values of 1/8 and 1/16 SM shall be reported Sector visibility shall be reported in remarks Significant Clouds. Significant clouds as described in Chapter 9 shall be evaluated and reported Runway Condition. The runway condition report as supplied by ATC personnel will be reported and encoded as described in Chapter

69 Additional Remarks. Enter any additional remarks deemed operationally significant. This includes information from local PIREPs reporting significant or unusual variations in the local weather that could affect flying operations, e.g., CLD LYR AT 004 FT ON APCH RWY 23 RPRTD BY PIREPS, CIG VIS LWR ON APCH RWY14L. 4.3 Tower Visibility Augmentations. A tower visibility report shall be made by the ATC personnel notifying the observer whenever the visibility is less than 4 miles. When a human observer is present, an augmentation for tower visibility shall be made. As described in Chapter 13, the lower value (if less than 4 miles) shall be reported in the body of the METAR, and the other value shall be reported in the remarks section. 4.4 Basic Operator Interface Device (OID) Operations The ASOS system uses a number of OID screens to display current weather and system data to the user. Access to various OID screens is via the auxiliary keypad. Display screens are defined as the output to the OID terminal as a result of invoking a primary ASOS function on the One-Minute Screen. The One- Minute Screen is the Figure 4-1 OID 1- Minute Edit Screen starting point when performing all procedures. Figure 4-1 shows the 1-Minute Screen with the observer menu keypad located in the lower right corner of the display. The menu keypad, which correlates to the auxiliary keypad of the OID keyboard, identifies the eight functions available. Detailed instructions on operator use of the OID are contained in the ASOS Ready Reference. Configuration procedures for the OID are contain in Appendix III of the ASOS Ready Reference Sign-On and Sign-Off Procedures. Signing onto ASOS allows privileged users to access the data processing procedures required for weather data manipulation. Signing off the system terminates the capability of the observer to manipulate weather data; however, weather data can be reviewed at any time without signing onto the system. In order to enter augmentation data into the OID, the observer must be signed on. The "AUTO" tag at the beginning of the observation will be dropped when the observer signs on. Sign-On and Sign-Off procedures are listed in Table 4-1. Any of the displays can be accessed by pressing the respective key on the keypad. 4-4

70 Table 4-1. Sign-On and Sign-Off Procedures Sign-on/Sign off When Entering SPECI. When using ASOS, several augmented events (e.g., tornadic activity, thunderstorm, or hail) automatically generate SPECI observations for the beginning and ending of the event. If one of these events is occurring at the close of augmentation coverage, it will be necessary to end the event or it will continue to be reported during the hours when there is no augmentation coverage. The observer shall end the event immediately after the last hourly METAR is transmitted before going off duty. The ending of the event will automatically generate a SPECI. The observer shall cancel this SPECI, enter the AUTO REMARK and disable the present weather (PREWX). The observer shall then sign off the automated weather observing system. This procedure will end the erroneous ending remark in the next observation Special Keys. There are several special keys on the OID keypad that perform functions that are critical to the operation of the ASOS system by any user; these keys and functions are listed in Table 4-2. Table 4-2 Special Function Keys 4-5

71 4.4.4 Alarms. The visual alarm device for ASOS is called the Observer Notification Device (OND), which is a light that flashes when a report (i.e., METAR or SPECI) has been automatically generated by ASOS. The OND also flashes when system problems are detected. ASOS constantly monitors for conditions that invoke audible alarms. An audible alarm sounds at the Operator Interface Device (OID) when any of the following conditions exist: An hourly routine report (METAR), special report (SPECI), or a local alert is automatically generated. The sensor field of the 1-Minute Screen indicates that the associated sensor is set to manual mode and has not been edited since the transmission of the last METAR. The data quality of a weather parameter is outside the acceptable range. 4-6

72 CHAPTER 5 BACK-UP OBSERVATION PROCEDURES FOR ASOS This chapter presents the general procedures and practices for providing the backup weather information required in the event of a partial or total failure of ASOS or other automatic sensor system. It also is applicable if one or more of the elements within any automated weather observing system observation are judged to be erroneous or nonrepresentative. Specific observation procedures for each element are contained in the manual observation section of this manual. Responsible personnel shall provide the backup weather information. Activities designated to perform augmentation shall also augment during periods when backup is required. 5.1 Back-up Equipment. A list of backup equipment required for all METOC service level A, B or C stations is provided in paragraph and NAVMETOCCOMINST (). There is no back-up equipment for the observation of visibility, present weather (except precipitation amount) and sky condition as these elements are visually observed. Use of back-up equipment is described in the Manual observations sections. 5.2 Back-up Actions. If the ASOS or any automatic observation system become inoperative, or in the judgment of the observer, the report is not representative of current conditions, or a specific sensor appears to be giving incorrect information, the observer should intervene by backing up the report. The observer should determine the appropriate action to be taken depending on the assessment of the situation. The observer must also be cognizant of the characteristics of ASOS (e.g., use of time averaging vs. spatial averaging) and the siting of the system when determining unrepresentativeness. The observer must use discretion in determining whether to turn off report processing for a sensor. Turning off report processing creates a maintenance indicator ($) and places it in the next observation. The observer will revert to manual observation procedures for any element not being properly sensed and/or reported by ASOS, and therefore should refer to the appropriate chapter for manual observation of that element ASOS Partial Failure (one, two, etc sensors or calculations). The observer will perform manual observation procedures only for the missing data, edit the ASOS observation by entering the manual data and calculations via the OID, and transmit the observation normally ASOS Communications Failure. The observer shall record the data from the ASOS and manually transmit the observation ASOS Total Failure. The observer will perform all manual observation procedures as outlined in Chapters 6 through 12, record and transmit the observation Suspension of Augmented Observations. If an observer who has been partially backing up present weather and/or entering remarks goes off duty, that observer must end the present weather and remarks; otherwise, he/she will continue throughout the period of unaugmented/back-up operation. 5-1

73 5.2.5 Observer Priorities. To perform backup duties, the observer shall remain aware of current weather conditions and any elements being reported by automatic sensor system. In the case of conflicting responsibilities or tasks, the observer shall use the posted duty priorities to determine which takes precedence, with the highest priority being safety of flight Recording Backup Observations. Backup observations shall be recorded on Surface Weather Report Form (CNMOC 3140/12). Use and disposition of this form are provided in Chapter

74 CHAPTER 6 - MANUAL OBSERVATIONS - GENERAL PROCEDURES This chapter prescribes general procedures applicable to the taking of manual surface weather observations for all weather elements. Individual chapters define the procedures and practices to be followed in observing and reporting specific elements of the surface-based meteorological conditions. The procedures outlined in this chapter and the specific elements chapters also apply to manual observations taken to fulfill requirements for augmentation or backup. 6.1 The Observational Process. The observation is the end product of a two or three step process. First, the weather elements must be observed and scientifically evaluated. Elements normally evaluated are wind, visibility, weather (precipitation, thunderstorms, tornadoes), obstructions to visibility, sky condition, temperature, dew point, and pressure. When done manually, data concerning these elements are then recorded in a standard format. Finally this data is encoded into a World Meteorological Organization (WMO) standard code recognized by all meteorological personnel worldwide. 6.2 Weather Watch and Observer Responsibility. Observers shall monitor weather conditions via a weather watch. Navy and USMC stations do not usually have a continuous weather watch, instead relying on a Basic Weather Watch. During a Basic Weather Watch, the observer may be required to perform other duties as their observing workload permits. The observer shall check the outside conditions as often as possible and be alert to situations conducive to significant changes in weather conditions. They shall also take and disseminate SPECI observations as rapidly as feasible whenever changes are noted that meet the SPECI criteria. Rapidly changing conditions require more frequent checks. 6.3 Types of Aviation Weather Reports. As discussed in Chapter 2 there are two major types of surface weather observation reports that satisfy the requirements for aviation weather: METAR and SPECI Aviation Routine Weather Reports - METAR. The METAR is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. These observations shall report wind, visibility, weather and obstructions to visibility, sky condition, temperature, dew point, pressure and any other phenomena deemed operational significant. Various other data are calculated from the basic observation information Aviation Selected Special Weather Reports - SPECI. A SPECI observation is an unscheduled observation taken when any of the criteria listed in Table 2-1 have been observed. A SPECI observation shall contain the elements in a METAR, plus additional coded or plain language information that elaborates on the data in the body of the report. All SPECI shall be taken as soon as possible after relevant criteria are observed. The element causing the SPECI shall be observed last. 6-1

75 6.4 Encoding of the Observations. These reports are encoded using the METAR or SPECI codes. Encoding of the observations is discussed in Chapter 13 and summarized in table Procedures and Assumptions. All manual observations shall be taken, recorded and disseminated in accordance with Chapter 2 - General Procedures, the procedures prescribed in this chapter, and with the instructions contained in the chapters on each specific element. These procedures assume that METAR observations are taken hourly and that SPECI observations are taken whenever significant changes to critical weather are observed. Manual weather observations are recorded on Surface Weather Report Form (CNMOC 3140/12). The observations should reflect only those conditions seen or reliably reported from the usual point of observation and, unless otherwise specified, must have occurred within 15 minutes prior to the standard time of the observation. Temporary or mobile land stations shall also use this form to record their observations. 6-2

76 CHAPTER 7 - WIND OBSERVATIONS This chapter describes the procedures for observing and determining wind data for inclusion in surface weather reports. Wind is measured in terms of velocity, a vector that includes direction and speed. The absence of apparent motion of the air is termed CALM. The direction and speed of the wind should be measured in an unsheltered, unobstructed area. This will avoid, to a large degree, the measuring of wind directions and speeds disturbed by local obstructions and will result in the reporting of winds more representative of the general weather patterns and more representative for aircraft operations. 7.1 Definitions Direction of Wind. Wind direction is defined as the direction from which the wind is blowing in tens of degrees true Speed of Wind. Wind speed is the rate of horizontal flow of air past a given point, measured in whole knots Gusts. Rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls. Only reported if occurrence is within 10 minutes of the observation time Squall. A strong wind characterized by a sudden onset and duration on the order of minutes. It is often accompanied by a shower or thunderstorm. For reporting purposes, the term is applied to any sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least 1 minute Magnetic Variation. Magnetic variation is the angle between true north and magnetic north. It is either "East" or "West" according to whether the compass needle points to the east or west of the geographical meridian Peak Wind Speed. The highest instantaneous wind speed observed. For reporting purposes, the highest instantaneous wind speed observed that exceeds 25 knots since the last METAR Variable Wind Direction. The wind direction may be considered variable if during the 2-minute evaluation period, the wind is 6 knots or less, OR during the 2-minute evaluation period, the wind direction fluctuates by 60 degrees or more and the wind speed is greater than 6 knots Wind Shift. A change in wind direction of 45 degrees or more which takes place in less than 15 minutes and has sustained winds of 10 knots or more throughout the wind shift. 7-1

77 7.2 Wind Evaluation. Wind direction, speed, character, and wind shifts shall be determined at all stations. Peak wind shall be determined at all stations with a means of recording or holding the observed wind speed in memory Wind Direction. Wind direction can most easily be determined by analyzing the high-resolution ASOS data if available. If not available, obtain wind direction from a digital readout, a direct dial indicator or vane indicator by averaging the observed wind direction over a 2-minute interval Use of the Handheld Anemometer. At stations not having a permanently installed wind sensor system or where the system is inoperative, the observer shall use the handheld anemometer (AMES RVM 96B) or PMQ-3. Within the reasonable confines of the standard point of observation, the observer must select a location where nearby obstructions such as buildings, vegetation, etc. do not affect the wind reading. The observer aligns the instrument to Magnetic North using the small compass in the instrument, and then holding the instrument level observes the vane direction over a period of two minutes to obtain an average direction. The direction must then be converted to True. The instrument will show significantly more variation in direction than the permanently installed equipment since it has no buffering Direction Estimation. When no backup instrument is available, an estimated direction can be obtained by observing the field wind cone or tee, movement of twigs, leaves, smoke, etc., or by facing into the wind in an unsheltered area. When estimating wind direction, note that even small obstacles may cause variations in the wind direction. Do not use the movement of clouds to estimate the surface wind direction regardless of how low the clouds are Variable Wind Direction. The wind direction may be considered variable if, during the 2-minute evaluation period, the wind speed is 6 knots or less. Also, the wind direction shall be considered variable if, during the 2-minute evaluation period, it varies by 60 degrees or more when the average wind speed is greater than 6 knots Wind Speed. Wind speed can most easily be determined by analyzing the highresolution ASOS data if available. If this data is not available, obtain wind speed from a digital readout or use a direct dial indicator. The speed is determined by averaging the observed speed, to the nearest knot, over a 2-minute period Use of the Handheld Anemometer. At stations not having a permanently installed wind sensor system or where the system is inoperative, the observer shall use the handheld anemometer (AMES RVM 96B). Following the location requirement given in paragraph , observe the speed indicator on the instrument for a period of 2 minutes and mentally average the speeds observed. The instrument will show significantly more variation in speed than the permanently installed equipment since it has no buffering Estimation of Wind Speed. If no backup instrument is available, use the Beaufort Scale (Table 7-1) as a guide in estimating the wind speed. Estimate wind speed based on a 2-minute average. 7-2

78 Beaufort Scale # WIND EQUIVALENT -- BEAUFORT SCALE MPH KTS International Description Specifications 0 < 1 < 1 Calm Calm; smoke rises vertically Light Air Smoke drift indicates direction of wind; wind vanes still Light Breeze Wind felt on face; leaves rustle; vanes moved by wind Gentle Breeze Leaves and small constantly moving wind twigs in motion; extends light flag Moderate Raises dust, leaves and Breeze loose paper. Small branches move Approximate Perpendicular force in pounds/sqft Fresh Breeze Small trees in leaf sway Strong Breeze Large tree branches in motion; whistling heard in wires; umbrellas used with difficulty Near Gale Whole trees in motion; resistance felt walking against the wind Gale Breaks twigs off trees; impedes progress Strong Gale Slight structural damage occurs Storm Trees uprooted; considerable structural damage occurs Violent Storm Widespread damage; many objects airborne Hurricane Widespread damage, structural failure of unreinforced wooden buildings Table 7-1. Estimating Wind Speed Beaufort Scale

79 7.2.3 Gusts. The speed of a gust is the maximum instantaneous wind speed observed during the most recent 10 minutes of actual time of report. The existence of gusts can most easily be determined by analyzing the high resolution ASOS data if available, and looking for a 10-knot difference. If this data is not available, gusts can be determined by carefully observing a digital readout or direct dial indicator. The observer must watch for the peaks and lulls and determine if there is a 10-knot difference. At stations not having a permanently installed wind sensor system or where the system is inoperative, the observer shall use the handheld anemometer (AMES RVM 96B) or PMQ-3. If gusts are suspected to be occurring, observe the speed indicator on the instrument for a period of 5 minutes to determine if gusts are occurring. The existence of gusts cannot be estimated using the Beaufort Scale Squalls. The existence of squalls can most easily be determined by analyzing the high resolution ASOS data if available, and looking for sudden increase of wind speed of at least 16 knots and is sustained at 22 knots or more for at least 1 minute. Successive observations using the handheld anemometer over a period of time could be used to determine the occurrence of a squall. The occurrence of squalls cannot be estimated using the Beaufort Scale Peak Wind Speed. Since peak wind speed is defined as the highest instantaneous speed recorded greater than 25 knots since the last METAR or essentially over an hour, it can only be determined if there is constant monitoring. A review of the high-resolution ASOS or other automatic sensor data is the only way to determine peak wind Wind Shifts. A wind shift is indicated by a change in wind direction of 45 degrees or more in less than a 15-minute period, with sustained wind speeds of 10 knots or more throughout the wind shift. The time of the wind shift is the time the shift began; however, it can not be reported until the criteria of 45 degrees has been met. Wind shifts are normally associated with some or all of the following phenomena: Gusty winds shifting in a clockwise manner in the Northern Hemisphere (counterclockwise in Southern Hemisphere). Rapid drop in dew point Rapid drop in temperature Rapid rise in pressure In summer: lightning, thunder, heavy rain, and hail In winter: frequent rain showers or snow showers Examination of the high resolution ASOS data or other automatic sensor data, if available, along with observation of the above phenomena is the most common method to determine a wind shift. If the high-resolution data is not available, use of the handheld anemometer at frequent intervals when the above phenomena are observed will provide a reliable indication of a wind shift. A SPECI shall be taken immediately after a wind shift occurrence, and a remark, reporting the wind shift and the time the wind shift occurred shall be included in the report. When the shift is believed to be associated with a frontal passage, report "FROPA" in remarks immediately after the time the shift began. 7-4

80 7.3 Conversion of True to Magnetic Winds. Obtain the local variation from an aeronautical chart and proceed as follows: To convert from true to magnetic direction: Add westerly variation to true direction; subtract easterly variation from true direction To convert from magnetic to true direction: Add easterly variation to magnetic direction; subtract westerly variation from magnetic direction. 7.4 Priority of Instruments. At stations having several types of equipment, observe the following priority in selecting the wind equipment to be used: High resolution ASOS data or ASOS observation Other automated meteorological sensor data Digital readout. Direct-reading dials Handheld anemometer (AMES RVM 96B) or PMQ Conversion of Wind Speed. Tables 7-2 and 7-3 are to be used as the official conversion of miles per hour to knots and knots to miles per hour. CONVERSION OF MILES PER HOUR TO KNOTS MPH KTS KTS KTS KTS KTS KTS KTS KTS KTS KTS Note: This figure is not reversible. Use Table 7-3 to convert knots to miles per hour Table 7-2. Conversion of Miles per Hour to Knots 7-5

81 CONVERSION OF KNOTS to MILES PER HOUR KTS MPH MPH MPH MPH MPH MPH MPH MPH MPH MPH Note: This figure is not reversible. Use figure 7-2 to convert miles per hour to knots Table 7-3 Conversion of Knots to Miles per Hour 7-6

82 CHAPTER 8 - VISIBILITY OBSERVATIONS This chapter describes the procedures for observing and determining visibility for inclusion in surface weather reports. Visibility is a measure of the opacity of the atmosphere and is expressed in terms of the horizontal distance at which you are able to see and identify specified objects. All visibilities referred to in this chapter are horizontal visibilities. An automated instrumentally derived visibility value is a sensor value converted to an appropriate visibility value using standard algorithms and is considered to be representative of the visibility in the vicinity of the airport runway complex. A manually observed visibility value is obtained using the "prevailing visibility" concept. 8.1 Definitions Prevailing Visibility. The greatest visibility equaled or exceeded throughout at least half the horizon circle, which does not necessarily have to be continuous. This definition applies to both surface and tower prevailing visibility Surface Visibility. The prevailing visibility determined from the usual point of observation Tower Visibility. The prevailing visibility determined from the control tower. With the advent of significantly higher control towers there is an increased likelihood that this visibility will be different from the surface visibility Sector Visibility. The visibility in a specified direction that represents at least a 45-degree arc (portion) of the horizon circle Variable Prevailing Visibility. A condition when the prevailing visibility is less than 3 miles and rapidly increasing and decreasing by ½ SM or more during the period of observation Visibility Markers. Dark or nearly dark objects viewed against the horizon sky during the day or unfocused lights of moderate intensity during the night, of a measured distance from the usual point of observation. 8.2 Visibility Observation Standards and Requirements Location of Observations. Visibility may be determined and reported at the surface, the usual point of observation, the tower level, or both. Tower visibility will be evaluated whenever the prevailing or tower visibility is four (4) miles or less and the tower visibility differs from the prevailing visibility. Within the reasonable confines of the standard point of observation, the visibility observations shall be taken from several viewpoints at one location as necessary to view as much of the horizon as practical. In this respect, natural obstructions, such as trees, hills, etc., are not obstructions to the horizon. These natural obstructions define the horizon. 8-1

83 8.2.2 Unit of Measure. In the U.S., visibility shall be reported in statute miles or fractions thereof. Table 8-1 lists all reportable visibility values for manual observations. OCONUS visibility is reported in meters. If visibility falls halfway between two reportable values, the lower value shall be used. Reportable visibility values for manual observations differ slightly from ASOS reportable values. ASOS Reportable Visibility Values Manual. Miles Meters Miles Meters Miles Meters Miles Meters Miles Meters M ¼ / / ¼ 2 ½ 1/16# 0100 ¾ ¾ ½ 3 1/8# / / ¾ 4 3/16# ¼ / ¼ ¼ 7 5/ ¼ ½ ½ 10 3/ / ¾ ¾ 1/ / * 9999 Notes: * Further increments of 5SM may be reported, i.e., 40, 45, 50, etc. # Visibility values of 0, 1/16, and 1/8 can be augmented in the visibility field of ASOS. Table 8-1 Reportable Visibility Values Observing Aids for Visibility. Visibility marker locations shall be readily available for quick reference. Panoramic pictures are outstanding reference tools to assist the observer and tower personnel. All reference guides shall be annotated from the point of observation with the direction, distance from station, and day/night time usage. A Visibility Check Point Diagram (Figure 8-1) shall be posted with the panoramic picture. The aids shall be labeled D and N to indicate whether they are day or night time markers. If tower visibility is reported, separate charts or lists of markers using the tower as an observation site shall be posted in the tower. The red or green course lights, television and radio tower obstruction lights etc., may be used as night time visibility markers. Because of their intensity, focused lights such as airport beacons shall not be used as markers. 8-2

84 W S Figure 8-1 Visibility Check Point Diagram "H" indicates object height in feet. 8-3

85 8.2.4 Dark Adaptation. Before taking visibility observations at night, the observer should spend as much time as practical in the darkness to allow the eyes to become accustomed to the limited light Control Tower Visibility Observations. Most stations having a control tower require ATC personnel to also take visibility observations Actions required of ATC personnel: Notify the weather station personnel when they observe tower prevailing visibility to decrease to less than, or increase to equal or exceed, 4 SM When the prevailing visibility at the tower or the surface is less than 4 miles, report all changes of one or more reportable values to the weather station personnel As required by FAA directives and as stated in paragraph 4.3, use the lower of either the tower or the surface visibility for aircraft operations Actions required of METOC personnel: Notify the tower as soon as possible, whenever the prevailing visibility at the surface observation point decreases to less than, or increases to equal or exceed 4 SM Re-evaluate prevailing visibility, as soon as practical, upon initial receipt of a differing control tower value, and upon receipt of subsequent reportable changes at the control tower level Encode the tower visibility in the METAR or SPECI. The tower visibility will be reported in the body of the report if it is lower than the surface visibility, and the surface visibility will be reported in the remarks. If the surface visibility is lower, it will be reported in the body of the report and the tower visibility reported in the remarks. 8.3 Evaluating Visibility. Visibility shall be evaluated as frequently as practicable. Using all available visibility markers, determine the greatest distances that can be seen in all directions around the horizon circle. When the visibility is greater than the distance to the farthest markers, estimate the greatest distance you can see in each direction. This estimate shall be based on the appearance of all visibility markers. If they are visible with sharp outlines and little blurring of color, the visibility is much greater than the distance to them. If a marker can barely be seen and identified, the visibility is about the same as the distance to the marker Determining Prevailing Visibility. After visibilities have been determined around the entire horizon circle, resolve them into a single value for reporting purposes. To do this, use either the greatest distance that can be seen throughout at least half the horizon circle, or if the visibility is varying rapidly during the time of observation, use the average of all observed values. 8-4

86 8.3.2 Sector Visibility. When the visibility is not uniform in all directions, divide the horizon circle into arcs (sectors) that have uniform visibility and represent at least one eighth of the horizon circle (45 degrees). The greatest visibility that prevails over at least half of the horizon circle, not necessarily continuous, is the prevailing visibility. For instance, if the visibility is ¼ to the NE, 1 to the SE, ½ to the SW and ¾ to the NW the prevailing is ¾. The other sectors shall be reported in the remarks of weather reports when they differ from the prevailing visibility by one or more reportable values and either the prevailing or sector visibility is less than 3 miles Variable Prevailing Visibility. If the prevailing visibility rapidly increases and decreases by one or more reportable values during the period of evaluation and the prevailing is less than 3 miles, the visibility is considered to be variable. The average of all values observed will be reported as prevailing and the minimum and maximum visibility values observed shall be reported in remarks. 8-5

87 CHAPTER 9 - WEATHER PHENOMENA OBSERVATIONS This chapter contains instructions on identifying, recording, and reporting present weather. For the purpose of this manual, present weather is defined as precipitation, obscuring phenomena, thunderstorms and tornadic activity. Some phenomena can be totally evaluated by ASOS sensors or other instruments; others require both instruments and manual observations, and some are only sensed manually. 9.1 Precipitation. Precipitation is any water particles that fall from the atmosphere and reach the ground, technically hydrometers. There are three forms of precipitation matching the three states of water: liquid, freezing and frozen. Liquid is any precipitation that does not fall as frozen precipitation and does not freeze upon impact. Freezing precipitation is any precipitation that freezes upon contact and forms a layer of ice (glaze) on the ground or on exposed objects. Freezing precipitation occurs when the surface temperature, or a layer of air aloft is warmer that 32 o F, but the surface and objects on the surface are below 32 o F. Frozen precipitation is any precipitation that reaches the ground in solid form. Solid precipitations can be crystalline or firm, opaque or clear ice Drizzle (DZ). Uniform precipitation composed exclusively of fine drops (diameter less than 0.02 inch or.5 mm) very close together. Drizzle appears to float, following the air currents, although unlike fog it does fall to the ground Rain (RA). Precipitation of liquid water particles generally in the form of drops larger than 0.02 inch or.5 mm. In contrast to drizzle droplets, raindrops are widely separated and a fall rate is readily discernable Freezing Drizzle (FZDZ). Drizzle that freezes upon impact with the ground or other exposed objects Freezing Rain (FZRA). Rain that freezes upon impact with the ground or other exposed objects Snow (SN). Frozen precipitation composed of opaque crystals, mostly branched in the form of six-pointed stars. At temperatures higher than about minus 5 C (23 F), the crystals are generally clustered to form snowflakes Snow Grains (SG). Precipitation of very small, white, opaque grains of ice. When the grains hit hard ground, they do not bounce or shatter. They usually fall in small quantities, mostly from stratus type clouds, and never as showers Small Hail or Snow Pellets (GS). Precipitation of white, opaque balls or cones of ice. The small hail may be mushy, sometimes called graupel. Snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm). 9-1

88 9.1.8 Ice Pellets (PL). Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch/5 mm, or less. The pellets usually rebound when striking hard ground, and make a sound on impact. Like freezing precipitation, ice pellets often occur when there is a warm layer of air aloft then the liquid rain falls through the below freezing layer, turning to ice before reaching the ground. There are two main types: Hard grains of ice consisting of frozen raindrops, or largely melted and refrozen snowflakes; often called sleet. This type falls as continuous or intermittent precipitation. Pellets of snow encased in a thin layer of ice which have formed from the freezing, either of droplets intercepted by the pellets, or of water resulting from the partial melting of the pellets. This type falls as showers Ice Crystals (IC). Unbranched ice crystals in the form of needles, columns, or plates. These are often so tiny that they seem to be suspended in the air. They may fall from a cloud or from clear air. The crystals are visible mainly when they glitter in the sunshine or moonlight (diamond dust); they may then produce a luminous pillar or other optical phenomena. This hydrometer (rarely more than the lightest precipitation), which is frequent in Polar Regions, occurs only at very low temperatures in stable air masses Hail (GR). Precipitation in the form of small balls or other pieces of solid ice (hailstones) falling separately or frozen together in irregular lumps. Hailstones consist of alternate opaque and clear layers of ice in most cases. Unlike other frozen precipitation, hail is generally a warm weather phenomenon associated with thunderstorms, when raindrops are drawn into the upper portions of the storm, then freeze and fall. They may repeat this process several times growing in size each time. They can range from pea size to grapefruit size Unknown Precipitation. Term used by automated weather observing systems to characterize precipitation of an unknown type that cannot be identified any further by the system. 9.2 Obscurations. Any phenomenon in the atmosphere, other than precipitation, that reduces horizontal visibility. This is not to say that precipitation alone can not reduce visibility. As indicated in figures 9-2 and 9-4,snow, ice pellets, and drizzle can significantly affect visibility; even rain at sufficient intensity can reduce visibility. Except where noted, obscurations are reported when the prevailing visibility is less than 7 miles or considered operationally significant. The forms of obscurations are either water based (hydrometers) or Earth or Pollutant Based (lithometeors). The particles making up the obscuration may be suspended in the air or lifted from the surface by the wind. They may float for extended periods of time or fall back to the surface within a short time depending on the size of the particles and velocity of the wind Fog (FG). A visible aggregate of minute water particles (droplets) that are based at the earth's surface and reduces horizontal visibility to less than 5/8 SM (statute miles), and unlike drizzle, does not fall to the ground. 9-2

89 9.2.2 Mist (BR). A visible aggregate of minute water particles (droplets) which are suspended in the atmosphere, but are based at the earth's surface and reduce prevailing visibility from 5/8 SM up to and including 6 SM. Sometimes called light fog Freezing Fog (FZFG). A suspension of numerous super cooled water droplets and or ice crystals based at the earth's surface reducing visibility to less than 5/8 SM. The super cooled water droplets frequently freeze upon contacting a surface and form hoar frost. Build up of hoar frost can be quite extensive accumulating to several feet when there is a strong wind. A report of freezing fog does not necessarily mean that ice is forming on surfaces Blowing Snow (BLSN). Snow particles raised by the wind from the ground to a height of 6 feet or more sufficient to restrict horizontal visibility Spray (PY). An ensemble of water droplets torn by the wind from the surface of an extensive body of water, generally from the crests of waves, and carried up a short distance in the air in such quantities as to reduce the horizontal visibility at eye level (6 feet) Haze (HZ). A suspension in the air of extremely small, dry particles invisible to the naked eye and sufficiently numerous to give the air an opalescent appearance. This phenomenon resembles a uniform veil over the landscape that subdues all colors. Dark objects viewed through this veil tend to have a bluish tinge while bright objects, such as the sun or distant lights, tend to have a dirty yellow or reddish hue. When haze is present and the sun is well above the horizon, its light may have a silvery tinge. Haze particles may be composed of a variety of substances; e.g., dust, salt, factory pollutants residue from distant fires, volcanoes, or pollen. The particles are well diffused through the atmosphere Smoke (FU). A suspension of small particles in the air produced by combustion. This phenomenon may be present either near the Earth's surface or in the free atmosphere. When viewed through smoke, the disk of the sun at sunrise and sunset appears very red. The disk may have an orange tinge when the sun is above the horizon. Evenly distributed smoke from distant sources generally has a light grayish or bluish appearance. A transition to haze may occur when smoke particles have traveled great distances; for example, 25 to 100 miles or more, and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere Widespread Dust (DU). Fine particles of dust or sand suspended in the air by a duststorm or sandstorm that may have occurred at or far away from the station. Dust gives a tan or gray tinge to distant objects. The sun's disk appears pale and colorless, or may have a yellow tinge when viewed through dust Blowing Dust (BLDU). Blowing dust is dust raised by the wind to a height of 6 feet or more, sufficient to restrict horizontal visibility. When visibility decreases to 5/8 SM or less, this becomes a duststorm (DS). 9-3

90 Blowing Sand (BLSA). Blowing sand is sand raised by the wind to a height of 6 feet or more, sufficient to restrict horizontal visibility. When visibility decreases to 5/8 SM or less, this becomes a sandstorm Volcanic Ash (VA). Fine particles of rock powder (often black) that are blown out from a volcano and that may fall like precipitation or may remain suspended in the atmosphere for long periods. Ash in even small amounts presents a significant hazard to aircraft engines. 9.3 Other Required Reportable Phenomena. These phenomena can have a significant impact on flight operations and shall be reported Thunderstorm (TS). A local storm produced by a cumulonimbus cloud that is accompanied by lightning and/or thunder, usually with gusty winds, heavy rain, and sometimes, with hail, infrequently snow or sand and dust Lightning (LTG). Lightning is defined as any of the various forms of visible electrical discharge produced by thunderstorms. Four main types of lightning can be distinguished. Cloud to Ground lightning (CG) is lightning occurring between a cloud and the ground. In-Cloud discharges (IC) are a type of lightning that takes place within a thundercloud or cumulonimbus. Cloud to Cloud discharges (CC) are streaks of lightning reaching from one cloud to another. Air Discharges (CA) are streaks of lightning which pass from a cloud to the air, but do not strike the ground Tornadic Activity Tornado (+FC). A violent, rotating column of air, forming a pendant and touching the ground. Usually occurs from a cumulonimbus cloud. It nearly always starts as a funnel cloud, and is accompanied by a loud roaring noise not unlike that of a locomotive Funnel Cloud (FC). A violent, rotating column of air, which does not touch the surface, usually a pendant from a cumulonimbus cloud Waterspout (+FC). A violent, rotating column of air which forms over a large body of water, such as a bay, gulf, or lake, and touches the water surface Volcanic Eruption. A geophysical event that causes the spewing of fire, molten rock, ash, and gases to the surface of the earth or into the atmosphere. The eruption 9-4

91 may extend to many thousands of feet into the atmosphere. Ash in even small amounts presents a significant hazard to aircraft engines Virga. Wisps or streaks of water or ice particles falling out of a cloud but evaporating before reaching the earth's surface as precipitation. Virga is frequently seen trailing from altocumulus and altostratus clouds, but also is discernible below the bases of high-level cumuliform clouds from which precipitation is falling into a dry subcloud layer Well-developed Dust/Sand Whirl (PO). An ensemble of particles of dust or sand, sometimes accompanied by small litter, raised from the ground in the form of a whirling column of varying height with a small diameter and an approximately vertical axis Sandstorm (SS). Particles of sand that are carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground. Sandstorm is reported when visibility is reduced to between 5/8 and 5/16 statute mile. If visibility is less than 5/16 statute mile, then heavy sandstorm (+SS) is reported Duststorm (DS). A severe weather condition characterized by strong winds and dust-filled air over an extensive area; may be accompanied by thunder and lightning. Duststorm is reported when visibility is reduced to between 5/8 and 5/16 statute mile. If visibility is less than 5/16 statute mile, then heavy duststorm (+DS) is reported. 9.4 Qualifiers and Descriptors. Present weather qualifiers are words, abbreviations or signs used to further describe the phenomena or present weather. They may be used in various combinations Intensity. Intensity of precipitation is an indication of the amount of precipitation falling at the time of observation. It is expressed as light (-), moderate (no sign), or heavy (+). No intensity is assigned to hail (GR) and ice crystals (IC) Showery (SH). Abrupt changes in precipitation intensity, or the precipitation starts and stops abruptly. The showery descriptor shall only be appended to rain (RA), snow (SN), ice pellets (PL), small hail/snow pellets (GS), or hail (GR) Continuous. Intensity changes little Intermittent. Precipitation stops or starts at least once during the hour and intensity changes gradually as opposed to quickly for showery Proximity. Used to describe locations of various phenomena On Station. Overhead or within 5 miles Vicinity. (VC) Indicates the phenomenon is occurring 5 to 10 miles from the station. 9-5

92 Distant (DSNT). Indicates the phenomenon is occurring at a distance of greater than 10 SM from the station Direction. A direction can be assigned to an observed phenomenon using an 8- point compass, i.e. N, NE, E, SE, S, SW, W, NW Shallow (MI). Used to describe fog when fog covers the station and visibility at 6 feet above the ground is 7 SM or more and the apparent visibility in the fog layer is less than 5/8 SM Partial (PR). When fog covers a substantial part of the station, visibility in the fog is less than 5/8 SM, and visibility over the uncovered parts of the station is 5/8 SM or more, "partial" shall be used to further describe the fog Patchy (BC). When fog covers portions of the station and the apparent visibility in the fog patch or bank is less than 5/8 SM and visibility over the uncovered portions of the station is 5/8 SM or greater, "patches" shall be used to further describe the fog. Both Partial and Patchy shall only be used to further describe Fog (FG) that has little vertical extent, normally greater than 6 feet but less than 20 feet and reduces horizontal visibility, but to a lesser extent vertically. The stars may often be seen by night and the sun by day. PR and BC may or may not reduce prevailing visibility Low Drifting (DR). Used to further describe the condition of dust, sand, or snow that is raised by the wind to a height of less than 6 feet and does not reduce the prevailing visibility Blowing (BL). Used to further describe the condition of dust, sand, snow, and/or spray which is raised by the wind to a height of 6 feet or more and does reduce the prevailing visibility. 9.5 Other Phenomena and Miscellaneous Terms. Most of these phenomena are not required to be reported, but are useful for helping to identify other reportable phenomena or characterizing the state of the atmosphere. Other terms are unique to the reporting methods for the phenomena Glaze (Clear Ice). The coating of ice, generally clear and smooth, formed on exposed objects at temperatures below or slightly above freezing by the freezing of super-cooled drizzle or rain drops. Glaze is extremely hazardous to flight and other operations and may be exceptionally destructive to trees and power lines when heavy Rime. A deposit of opaque ice, produced by super-cooled fog or cloud droplets (smaller than rain or drizzle) contacting objects at temperatures below freezing. It is composed of grains separated by air, sometimes adorned with crystalline branches Water Equivalent. This is the amount of liquid produced by melting frozen precipitation. 9-6

93 9.5.4 Core Sample. A core sample is a section cut from the snow cover at a station to determine the amount of water present in the solid state Snowboard. An aid for measuring new snowfall. The snowboard is made of a piece of thin, light-colored, wooden board, at least 2 feet square, or an equivalent, lightweight, poor conductor of heat Rainbow. Colored, circular (or nearly circular) arcs formed by light (usually sunlight) falling on a population of water drops such as provided by rain, cloud, fog, or spray. Rainbows are seen as related groups of arcs, such as the primary rainbow (with red on the outside, blue on the inside), the (larger) secondary rainbow (with red on the inside) Halo. A set of 22 degree whitish rings, arcs, pillars or spots of light that appear in the sky and are caused by the refraction of the light by ice crystals, usually found in the vicinity of the light source, the most important of which are the sun and moon. Color, if any, is usually fairly pale, being best on a red edge next to the light Corona. A set of one or more colored rings of small angular radii concentrically surrounding the sun, moon, or other light source caused by the diffraction of water vapor such as that in a thin cloud. The corona can be distinguished from the halo of 22 due to its much smaller angular radius, which is often only a few degrees, and by its distinct color sequence, which is from bluish white on the inside to reddish on the outside, the reverse of that in the 22 halo Crepuscular Rays. Literally twilight rays, these alternating dark and light bands (shadows and light scattered from sunbeams, respectively) seem to diverge fanlike from the sun's position during twilight. This apparent divergence of parallel sunlight is an artifact of linear perspective. Crepuscular rays may appear as 1) shadows cast across the purple light by high, distant cloud tops or 2) shadows next to light scattered from sunbeams by haze in the lower atmosphere. Sunbeams seen during the day are sometimes called crepuscular rays, even though they are observed outside twilight. 9.6 Evaluating Precipitation. Besides recognizing the type of precipitation, the observer must note times and determine the intensity, and character. If an ASOS is not installed, or if the ASOS precipitation identifier sensor can not identify the type of precipitation occurring, or if the sensor is inoperative, the observer shall visually determine and report the type of precipitation by using the definitions in this chapter. If the observer is still unsure as to the type occurring, refer to the Glossary of Meteorology, on-line at Precipitation observed at a distance from the station shall be reported in the remarks section Times. The observer shall note to the nearest minute the time that precipitation of any type is observed to begin and end. The time of any change in intensity shall be noted for freezing rain, freezing drizzle, or ice pellets. The beginning or ending time for hail, freezing precipitation, or ice pellets is the time used for the required SPECI; the beginning/ending time shall also be included in the following METAR. Times for 9-7

94 separate periods shall be reported only if the intervening time of no precipitation exceeds 15 minutes Character. Determine the character as continuous, intermittent, or showery. Showery is only associated with rain, snow, ice pellets, small hail or hail Intensity. Assuming the tipping bucket rain gauge is inoperative, the observer may estimate the intensity using a variety of means. The type of precipitation determines the method used Rain, or Freezing Rain. The intensity of rain or freezing rain shall be estimated using the visual effects given in Table 9-1. INTENSITY Light Moderate Heavy CRITERIA From scattered drops that, regardless of duration, do not completely wet an exposed surface, up to a condition where individual drops are easily seen. Individual drops are not clearly identifiable; spray is observable just above pavements and other hard surfaces. Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of several inches is observed over hard surfaces. Table 9-1 Estimating Intensity of Rain or Freezing Rain Ice Pellets. The intensity of ice pellets shall be estimated using the visibility criteria guidelines given in table 9-2. INTENSITY Light Moderate Heavy CRITERIA Scattered pellets that do not completely cover an exposed surface, regardless of duration. Visibility is not affected. Slow accumulation on ground. Visibility reduced by ice pellets to less than 7 miles Rapid accumulation on ground. Visibility reduced by ice pellets to less than 3 miles. Table 9-2 Estimating Intensity of Ice Pellets 9-8

95 Rate of Fall. The intensity of rain, freezing rain and ice pellets can also be estimated on a rate of fall basis using Table 9-3. Freezing rain and ice pellets must be melted to determine liquid equivalent and since this estimation is determined over a period of time the intensity is not necessarily the intensity at the time of the observation. INTENSITY CRITERIA Light Up to 0.10 inch per hour; maximum 0.01 inch in 6 minutes. Moderate 0.11 inch to 0.30 inch per hour; more than 0.01 inch to 0.03 inch in 6 minutes. Heavy More than 0.30 inch per hour; more than 0.03 inch in 6 minutes Table 9-3 Intensity of Rain or Ice Pellets Based on Rate of Fall Snow and Drizzle. The intensity of snow or drizzle can be estimated based on visibility (Table 9.4). If snow is increasing at the rate of 1 inch or more per hour it shall be noted and reported in the observation. INTENSITY Light Moderate Heavy CRITERIA Visibility > 1/2 mile. Visibility > 1/4 mile but < =1/2 mile. Visibility <= 1/4 mile. Table 9-4 Intensity of Snow and Drizzle Based on Visibility Multiple Precipitation Types and Obscurations. When more than one form of precipitation is occurring at a time or precipitation is occurring with an obscuration, the observer must make a best estimate as to the intensities of the individual precipitation types and the affect of any obscuration. The intensities determined shall be no greater than that which would be determined if any of the forms were occurring alone Amount. Precipitation measurements are normally made by the automatic precipitation measuring device (ASOS heated tipping bucket), but can also be accomplished using the standard 4-inch plastic rain gauge. If more than one type of gauge is available, use the automated instruments first, then the 4-inch plastic rain gauge. Expressed in terms of vertical depth, precipitation measurements shall be made in inches, tenths of inches, or hundredths of an inch depending on the type of precipitation being measured (see Table 9-5). 9-9

96 TYPE OF PRECIPITATION Liquid or Freezing Solid (Actual Fall) Liquid Equivalent of Solid Total Snow depth on Ground INCREMENT.01 inch.1 inch.01 inch 1 inch Table 9-5 Precision of Precipitation Measurements Using ASOS. ASOS will report the liquid precipitation as it occurs to the nearest hundredth of an inch. It will also encode the precipitation amount in the hourly observation, at other times as required, and maintain summary data. If extremely heavy rainfall has occurred, the observer shall compare the ASOS reading with the 4-inch plastic rain gauge amounts since the ASOS tipping bucket has some known inaccuracy during these precipitation events. (See para ) Using the Four-Inch Plastic Rain Gauge Check daily to ensure no foreign debris are in the gauge Normally, the inner-measuring tube is emptied at 3 and 6-hourly synoptic times and at midnight LST; the amount collected is recorded at these times. The gauge may be emptied more frequently if necessary for local purposes, provided a record of precipitation amounts is maintained for use in determining the total amounts for applicable 3 and 6-hourly and midnight LST observations The inner-measuring tube of the 4-inch plastic rain gauge is calibrated to hundredths of an inch based on the 4-inch diameter of the outer gauge. Read the precipitation amount to the nearest hundredth from the markings on the inner measuring tube Read the level of water at the bottom of the meniscus If the inner-measuring tube has overflowed, pour an amount into the outer gauge so there is an even amount in the measuring tube, i.e. one inch, then empty the intermeasuring tube and pour the excess water in the outer gauge into the inner tube for measurement When freezing or solid precipitation is expected, remove the funnel and the one-inch inner measuring tube Determining Water Equivalent for Frozen/Freezing Precipitation. Although the ASOS tipping bucket rain gauge usually melts freezing and frozen precipitation when falling, essentially converting it to liquid equivalent; it is not the most accurate measurement for a variety of technical reasons. There is no ASOS sensor for determining water equivalent for overall snow cover. There are three manual methods for determining water equivalent. 9-10

97 Using the Overflow Container. If the frozen or freezing precipitation collected in the overflow container is considered representative, determine the water equivalent by adding a measured quantity of warm water to the overflow tube containing the solid precipitation in order to melt the contents. Pour the liquid into the measuring tube; obtain a measurement, and subtract an amount equal to that of the warm water added. The result is the actual precipitation amount measured to the nearest 0.01inch (i.e., the water equivalent of the frozen/freezing precipitation). If the collection in the overflow tube is considered unrepresentative (e.g., due to strong winds), discard the catch and obtain a measurement by means of vertical core sampling or by estimation Core Sampling for Water Equivalent of Frozen/Freezing Precipitation. A core sample is a section cut from the snow/ice cover to determine the amount of water present in the solid state. Select, for core sampling and depth measurements, an area that is smooth, level, preferably grass covered, and as free from drifting as possible. Paved areas and low spots where water tends to collect should be avoided. Snowboards should be placed on top of a previous snowfall. The size and utilization of the area should permit samples and measurements to be taken in undisturbed snow, approximately two feet apart. The deeper the snow and the greater the drifting, the greater the distance between samples to obtain representative measurements. Irregularities caused by uneven terrain, drifting, footsteps, prior sampling, etc. usually introduce some unavoidable errors in this type of water equivalent measurement. Invert the overflow container or the four-inch gauge over the top of the snow/ice pack and lower it to a snowboard or other reference point for the new snowfall (cookie cutter style). Use the snowboard or other object to collect the sample within the container. Melt the collection to obtain the water content as explained above. Measure the snow depth, to tenths of an inch, at the spot where the core sample has been. Also, measure the snow depth at the most representative location available, to tenths of an inch. Determine the density of the snow by dividing the water equivalent by the depth by using the snow depth found in at the core site. Multiply the most representative snow depth the density of the core sample to obtain the adjusted water equivalent of the snow Estimating the Water Equivalent of Snow. When the water equivalent of snow cannot be accurately measured by melting or core sampling, estimate the water equivalent to the nearest 0.01 inch. Use Table 9-6 only as a guide in estimating the water equivalency of newly fallen snow. 9-11

98 MELT WATER EQUIVALENT (INS) NEW SNOWFALL (INS) Temperature (Deg F) 34 to to to to 10 9 to 0-1 to to -40 Trace Trace , This table can only be used in determining amounts of newly fallen snow. Packing and melting/refreezing have substantial effects on the density of snow pack and are not accounted for in this table. Table 9-6 New Snowfall to Estimated Melt Water Conversion 9-12

99 9.6.6 Measurement of Solid Precipitation. For the purposes of depth measurements, the term snow also includes ice pellets, glaze, hail, any combination of these, and sheet ice formed directly or indirectly from precipitation. Therefore, if snow falls, melts, and refreezes, the depth of ice formed will be included in depth measurements of snow Snowboards. As an aid in obtaining the measurement of new snowfall, snowboards may be placed on top of the snow after each measurement. Flag or mark snowboards in such a way that it is left undisturbed by non-weather personnel and found by the observer. Each new snowfall measurement can then be taken from the top of the snow to the snowboard Measurement of New Snowfall. Using a standard ruler, yard stick (graduated in inches) or other suitable measuring device, measure the depth in several locations, preferably at points where the snow has fallen and is undisturbed by the wind. If practical, make these measurements using snowboards or a surface that has been cleared of previous snowfall. If snow is increasing at the rate of 1 inch or more per hour, it shall be noted and reported in the observation. If the previous snowfall has crusted, the new fall may be measured by permitting the end of the ruler to rest on the crust. If a suitable spot is not available and snowboards are not in place, the snowfall amount may be obtained by measuring the total depth of snow and subtracting the depth previously measured. This will normally be an estimation due to the effects of melting, sublimation, etc. Determine the average of the several measurements to the nearest 0.1-inch. Consider the amount as estimated if there is any doubt as to its accuracy, e.g., due to melting, drifting, etc. When melting or settling has occurred between measurements, estimate the depth of new snow that would have collected if the melting or settling had not occurred. For instance, if several snow showers occur between observations and each melts before the following one occurs, the total snowfall for the period will be the sum of the maximum depth (measured or estimated) for each occurrence Measurement of Total Snow Depth. For the purpose of snow depth measurements, the term snow also includes other types of frozen precipitation (e.g., ice pellets, hail) and sheet ice formed directly or indirectly from precipitation. Obtain total depth of snow in conjunction with snowfall measurements. Using a standard ruler, yardstick or other suitable measuring device, measure the total depth in several locations, preferably at points where the snow is undisturbed by the wind. If the ground is covered with ice cut through the ice with some suitable implement and measure its thickness. Add the thickness of the ice to the depth of snow above the ice. When the snow has drifted, include the greatest and least depths in measurements from the representative area. For example, if spots with no snow are visible, use zero as one of the values. Obtain the average of the several measurements, to the nearest whole inch. Estimates of total depth may be obtained using snow stakes at units where this method is considered necessary and practical. In such cases, place several stakes in the most representative area available, i.e. where the snow is least likely to be disturbed within a few feet (or meters) of the stakes. Obtain an average depth by reference to graduated markings on the stakes. 9-13

100 Evaluating Hail. In addition to the other requirements for solid precipitation, all hail observations shall report the diameter of the largest hailstones in 1/4-inch increments. No intensity is assigned to hail. 9.7 Evaluating Obscurations. Whenever obscuring phenomena restricts visibility at the station to less than seven miles or if operationally significant, the type, and any qualifier shall be reported. Volcanic Ash shall always be reported. Drifting Snow, Dust and Sand shall be reported if operationally significant even though they do not restrict visibility. Shallow fog, partial fog, and patches of fog may be reported even though the prevailing visibility is 7 SM or more Determining Type of Obscuration. The ASOS has limited ability to determine the type of obscuration. The observer, when on duty, shall visually determine and report the type of obscuration by using the definitions in this chapter. If the observer is still uncertain as to the type of obscuration occurring, refer to the Glossary of Meteorology, on-line at Other weather elements such as dewpoint, wind, precipitation, etc. often aid in the determination of the type of obscuration. Table 9-7 provides guidance for selection of obscuring phenomena based on visibility and dew point depression Haze versus Mist. Differentiating between mist and haze can be difficult at times. A general rule is if the dew point spread is 0 to 2º C, consider the phenomenon Mist. If the spread is greater than 2º C consider the phenomenon to be Haze. Each situation is different, and there will be instances where the spread is 3 or 4º and there is both mist and haze. Prevailing Visibility >= 7 miles Guide to Selection of Obscuring Phenomena Criteria Prevailing Prevailing Prevailing Visibility < 7 Visibility < 7 Visibility < 5/8 miles, but = or > miles, but = or > mile; Dewpoint 5/8 mile; 5/8 mile; Depression = or Dewpoint Dewpoint < 2deg C Depression = or Depression < 2deg C > 2deg C Prevailing Visibility < 5/8 mile; Dewpoint Depression > 2deg C Possible Obscurations Volcanic Ash Mist (light fog) Haze Fog Sandstorm Shallow Fog Blowing Spray Dust Freezing Fog Duststorm Partial Fog Blowing Snow Smoke Blowing Snow Smoke Patchy Fog Volcanic Ash Volcanic Ash Blowing Spray Blowing Snow Drifting Snow Blowing Dust Volcanic Ash Blowing Spray Drifting Sand Blowing Sand Volcanic Ash Dust Whirl or Devil Blowing Snow Blowing Spray Table 9-7 Guide to Selection of Obscuring Phenomena 9-14

101 9.7.3 Obscuration Qualifiers. There are numerous qualifiers that may be appended to the obscuration type or reported in the remarks section of the observation. Many of these are operationally significant. Since visibility is one of the most important elements for the military operations, the observer should error on the side of caution. Besides remarks on sector visibility, and variable visibility, some examples to be included are fog dissipating or increasing, smoke drifting over the field, drifting snow, and obscurations at a distance from, but not at the station Volcanic Ash. Because of the extreme hazard to aviation operations, volcanic ash shall be reported in the body of the report whenever it is observed. Reporting volcanic ash is different from other obscurations because volcanic ash is reported even if the visibility is greater than 7 miles. Evaluate intensity, direction of drift and amount on the ground. 9.8 Evaluating Thunderstorms. The observer shall be alert for the occurrence of thunderstorms whenever towering cumulus or cumulonimbus clouds are present. Concurrent monitoring of the weather radar is beneficial. A thunderstorm with or without precipitation will be reported in the body and remarks of the observation when observed to begin, be in progress, or to end. Remarks concerning the location, movement, and direction (if known) of the storm will also be reported Beginning and Ending of a Thunderstorm. As noted in the ASOS chapter, the ASOS lightning detector senses electrical discharges to determine the presence of a thunderstorm. The human observer shall determine a thunderstorm to have begun at the station when thunder is heard, or overhead lightning is observed and the local noise level is such as might prevent hearing thunder. A thunderstorm is considered to have ended 15 minutes after the last occurrence of thunder, or when overhead lightning is no longer observed when the local noise level is such as might prevent hearing thunder. Times are noted to the nearest minute Location. The observer shall note the thunderstorm s direction from station using an 8-point compass, (if known), or overhead ( OVHD ). If more than 10 miles evaluate as distant ( DSNT ). Vicinity (VC) may be used for cells from 5 to 10 miles Movement. The observer shall note the direction toward which the storm is moving (if unknown, enter "MOVMT UNKN"). 9.9 Evaluating Lightning. When lightning is observed, the observer shall note the frequency, type, and location. Observers should not remain outside on roofs or near tall objects during the occurrence of lightning. Table 9-8 provides a guide for evaluating Lightning Type and Frequency. The location is evaluated as overhead (OVHD), distant (DSNT), or using an 8 point compass. 9-15

102 TYPE OF LIGHTNING TYPE CONTRACTION DEFINITION Cloud to Ground CG Lightning bolts can be seen transiting between the cloud and ground In Cloud IC Lightning can be seen lighting up the interior of a cloud Cloud to Cloud CC Lightning bolts can be seen transiting between clouds Cloud to Air CA Lighting bolts pass from the cloud to the air but do not strike the ground FREQUENCY OF LIGHTNING FREQUENCY CONTRACTION DEFINITION Occasional OCNL Less than 1 flash a minute Frequent FRQ 1 to 6 flashes a minute Continuous CNS More than 6 flashes a minute Table 9-8. Type and Frequency of Lightning 9.10 Evaluating Tornadic Activity. Because of the severe nature of tornadic activity, providing details on the phenomena is always secondary to the dissemination of the report on the occurrence of the phenomena Begins. A funnel cloud, tornado or waterspout shall be immediately reported including the time it was first observed. If the phenomena is not seen by the observer but reported by a reliable source (ATC personnel or police) the phenomena shall also be reported Ends. The time a funnel cloud, tornado, or waterspout ends or disappears shall also be noted. Include the direction last seen if known Other Details. Location and/or direction from the station as well as the direction towards which the phenomenon is moving should be reported Evaluating Volcanic Eruption. When observed, volcanic eruptions will be reported. Pre-eruption volcanic activity shall not be reported nor will a continuous eruption. The presence of volcanic ash will always be reported. The observer shall note the time of the eruption, size description, approximate height, and direction of movement of the ash cloud as well as any other pertinent data about the eruption. Example: MT AUGUSTINE VOLCANO 70SM SW ERUPTED LARGE ASH CLOUD EXTENDING TO APPROX FEET MOV NE. 9-16

103 CHAPTER 10 - SKY CONDITION OBSERVATIONS This chapter provides guidance on the evaluation of the condition or appearance of the sky. Automated stations shall have the capability to evaluate sky condition from the surface to at least 12,000 feet. Observers at manual stations and when augmenting ASOS shall evaluate all clouds and obscuring phenomena visible, i.e., the 12,000-foot restriction shall not apply. A complete evaluation of sky condition includes the type of clouds, obscuring phenomena, their stratification, amount, direction of movement, and the height of their bases. The WMO International Cloud Atlas, Volumes I and II, and the Abridged Atlas contain detailed instructions and photo-aids for identifying the various cloud forms. Additional aids may be used for identifying cloud forms (types) such as cloud code charts and training papers in surface observations. Commercial products are also available that describe cloud forms and types. Descriptions of obscuring phenomena are included in Chapter 9 Weather Phenomena Definitions Field Elevation. The officially designated field elevation of an airfield. Expressed in feet above mean sea level; it is calculated for the landing area (normally the highest point of the primary runway). For locations not associated with an airfield, it is the height of the normal point of observation Celestial Dome. That portion of the sky that would be visible provided there was an unobstructed view of the horizon in all directions from the observation site Horizon. The lower boundary of the observed sky or the upper outline of terrestrial objects, including nearby natural obstructions such as trees and hills. It is the distant line along which the earth (land and/or water) and the sky appear to meet Cloud. A visible accumulation of minute water droplets and/or ice particles suspended in the atmosphere above the Earth's surface. Clouds differ from fog or ice fog only in that the latter are, by definition, in contact with the surface of the earth Obscuring Phenomena. Any collection of particles, such as fog, haze and smoke, either aloft or in contact with the earth, which are discernible to the observer. The term is also applied to precipitation (e.g., rain, snow, drizzle) when it obscures part or all of the sky Vertical Visibility. The height that one can see upward into a surface-based obscuring phenomena (obscured sky), or the height corresponding to the upper limit of a laser beam ceilometer s reading Layer. Clouds and/or obscuring phenomena (not necessarily all of the same type) whose bases are at the same level. A layer may be either continuous or composed of detached elements. If present, a partly obscured condition is always considered the lowest layer. 10-1

104 Layer Amount. The amount of sky cover at a given level expressed in eighths Layer Height. The height, in feet, of the layer's base above the observation point surface or the vertical visibility into a surface-based obscuring phenomenon Layer Opacity. All cloud layers and obscuring phenomena shall be considered opaque Summation Principle. The basis by which sky cover classifications are made. This principle states that the sky cover at any level is equal to the summation of the sky cover of the lowest layer plus the additional sky cover present at all successively higher layers up to and including the layer being considered. No layer can be assigned a sky cover less than a lower layer, and no sky cover can be greater than 8/8. This concept is applicable for the evaluation of total sky cover as well as determining ceiling Sky Cover. A term used to denote the amount (to the nearest eighth) of the sky covered by clouds and/or obscuring phenomena, hidden by surface based phenomena or any combination of the two Sky Cover Classification. The terms used to reflect the degree of cloudiness in sky condition evaluations based on a summation of the amount of cloud cover or obscuring phenomena at and below the level of a layer aloft. There are seven basic sky cover classifications Clear. The term used to describe the absence of clouds or obscuring phenomena. For manual reports this indicates no clouds are present; for automated reports, this indicates that no clouds are detected at or below the design limit of the ceilometer Few. A summation sky cover of a trace through 2/8ths. Note that a trace of cloud or obscuration aloft is considered as 1/8th when it is the only layer present Scattered. A summation sky cover of 3/8ths through 4/8ths Broken. A summation sky cover of 5/8ths through less than 8/8ths. More than 7/8ths but less than 8/8ths is considered as 7/8ths for reporting purposes Overcast. A summation sky cover of 8/8ths Partly Obscured. A condition in which surface-based obscuring phenomena are hiding at least 1/8th, but less than 8/8ths, of the sky or higher layers. The term partial obscuration may also be used in relation to this sky condition Obscured. A condition in which surface-based obscuring phenomena (e.g., fog or snow) are hiding 8/8ths of the sky. The terms obscuration and indefinite ceiling may also be used in relation to this sky condition. 10-2

105 Ceiling. The ceiling is the height above the earth s surface at the observation point ascribed to the lowest broken or overcast non-surface based layer; OR the vertical visibility into a surface based obscuring phenomena that totally hides the sky Variable Ceiling. A term that describes a condition in which a ceiling, below 3000 feet, rapidly increases and decreases by one or more reportable values while the ceiling height is being evaluated Variable Sky Condition. A term used to describe a sky condition that has varied between reportable classifications (e.g., SCT to BKN, OVC to BKN) during the period of evaluation (normally the past 15 minutes) Total Amount. The amount in eighths, of the entire sky covered by all layers present. This amount cannot be greater than 8/ Cloud Movement. When reported, it is the direction towards which a cloud is moving Sky Cover Evaluation. The ASOS ceilometer should be used to evaluate sky conditions when operational. The observer may supplement or modify the sky condition as outlined in the following paragraphs. Sky condition observations require an evaluation of all atmospheric phenomena that prevent an uninterrupted view of the celestial dome, sun, moon, etc. Such phenomena generally occur in the form of layers with comparatively level bases. Sky condition will be evaluated in all METAR and SPECI observations. Evaluate all clouds and obscurations that are visible for amount and height of bases for each observation. When obscuring phenomena totally obscure the sky, an evaluation of the vertical visibility is made. For selected observations, the type of clouds and movement shall be made. For partial obscurations, an evaluation is made to determine the amount of sky obscured. Take observations from as many locations as necessary and practical to view the entire sky above the natural local horizon Evaluating Cloud Height. The height of clouds or the vertical visibility into a totally obscured sky is reported for all layers to the degree of accuracy given in Table Assuming the ceilometer is inoperative, there are several methods to objectively determine cloud height. With time, experience will allow the observer to reasonably estimate the height of clouds using only the eye. Range of Height Values (feet) Less than or equal to 5,000 feet Greater than 5,000 up to and including 10,000 feet Greater than 10,000 feet Reportable Values (feet) Nearest 100 feet Nearest 500 feet Nearest 1,000 feet Table 10-1 Cloud Height Reportable Values 10-3

106 Pilot Reports. Air Traffic Control should be routinely queried for reports from inbound aircraft. The height of the bases can be used as a guide when determining the cloud bases Known Heights of Fixed Objects. Many air facilities are surrounded by buildings, towers, and trees that may help determine the height of low cloud bases. Each observing station should maintain a locally prepared diagram annotated with the direction from the observation point, distance and height of fixed objects Skew-T Log P Diagram. The vertical structure of the atmosphere is displayed on the Skew T Diagram and therefore areas of moisture can easily be determined. The Lifted Condensation Level (LCL) and the Convective Condensation Level (CCL) can also be used to estimate bases Convective Cloud Height (Table 10-2). The air temperature and the dew point can be used to obtain the height of air mass convective cloud bases in the vicinity of the airfield. This convention is most accurate with cloud bases below 5,000 feet and it cannot be used in mountainous or hilly terrain. CONVECTIVE CLOUD-BASE HEIGHT TABLE Dew point Depression (ºC) Estimated Cloud Base height Dew point Depression (ºC) Estimated Cloud Base height , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,000 This table is not suitable for use at stations situated in mountainous terrain, and it should be used only when clouds are formed by active surface convection in the vicinity of the station. Table 10-2 CONVECTIVE CLOUD-BASE HEIGHT TABLE 10-4

107 Doppler Weather Radar. WSR-88D and Supplemental Doppler Weather Radar s vertical presentations can be evaluated for the height of cloud bases Other Station Reports. Surrounding airfield observations can assist in determining the cloud bases. Approaching frontal activity as well as fair weather clouds may be consistent by region Element Size. The apparent size of cloud elements, rolls, or features visible in the layer. Large rolls or elements greater than 5 degrees wide (three fingers is about 5 degrees when at arms length) usually indicate the layer is relatively low. Small rolls or elements between 1 and 5 degrees wide (the little finger is about 1 degree wide when at arms length) usually indicate the layer is relatively high Determination of Vertical Visibility. Vertical visibility shall be the distance that an observer can see vertically into a totally obscuring phenomenon. Some of the same techniques used to determine ceiling (heights of known fixed objects, pilot reports) can be used to determine vertical visibility. Additionally, observation of aircraft taking off and landing is especially useful Determination of Height at Night. The reflection of city or other lights at night can be used. At night, lights may reflect off the base of a layer. This could help estimate the low layer s height. For example, a cloud layer over a city to the west may be noticeably illuminated when the base is 5,000 feet or lower. However, a layer may be 1,000 feet or lower for any appreciable illumination from a small town to the northeast of the unit's location Evaluating Layers. All clouds at approximately the same height will be considered a layer. All layers visible from the station will be reported in sky cover reports. The report will be based on the eighths (or oktas) of sky covered by each layer in combination with any lower layers. The amount of sky cover for each layer will be the eighths of sky cover attributable to clouds or obscurations (i.e., smoke, haze, fog, etc.) in the layer being evaluated. Additionally, all layers composed of cumulonimbus or towering cumulus will be identified. The maximum number of reportable layers allowed in CONUS is six. ASOS provides only three. OCONUS is restricted to three reportable layers except when convective clouds (CB or TCU) are observed and not reported in the previous three layers. In such cases, a fourth layer may be used Multiple Layers. Frequent observations are necessary to evaluate stratification. A series of observations will often show the existence of multiple layers. Through thin lower layers it may be possible to observe higher layers. Differences in the directions of cloud movements often aid in observing and differentiating cloud layers Interconnected Cloud Layers. Cumuliform clouds may develop below other clouds and reach or penetrate them. These cumuliform clouds may also form Stratocumulus, Altocumulus, or dense Cirrus by horizontal extension. If these are attached to a parent cloud, they shall be regarded as a separate layer only if their bases appear horizontal and at a different level from the parent cloud. Otherwise, the entire 10-5

108 cloud system shall be regarded as a single layer at a height corresponding to that of the base of the parent cloud Evaluating Amount. Mentally divide the sky into halves or quarters and estimate the layer amount in eighths in each section. Add the total amount of eighths estimated from each quadrant to arrive at a celestial dome coverage estimate. The inverse of this procedure can be done by estimating the amount of clear sky. Subtract this amount from 8/8ths to obtain an estimate of layer coverage Estimating the Amount of an Advancing or Receding Layer. To estimate the amount of an advancing (or receding) layer which extends to the horizon, determine the angular elevation above the horizon of the forward or rear edge of the layer as seen against the sky. Convert the angle to a sky cover amount using the left side of Table When the layer does not extend to the horizon, determine the angular elevation of the forward and rear edges and the eighths of sky cover corresponding to each elevation angle. The difference will equal the actual sky cover Estimating a Partial Obscuration. If the partial obscuration is present in the form of a continuous layer surrounding the unit and extending to the horizon, determine the angular elevation of the edge of the layer and convert it to a sky cover amount using the right side of Table If the partial obscuration exists on only part of the horizon, the angular elevation can still be used, but the amount of coverage must be reduced in proportion to the amount of horizon affected. Angle of Advancing or receding Layer s Edge SKY COVER AMOUNT Eighths of Sky Cover Angle of Elevation of a Layer/Partial Obscuration Surrounding Station >0º to 50º 1 >0º to 10º 51º to 68º 2 11º to 17º 69º to 82º 3 18º to 24º 83º to 98º 4 25º to 32º 99º to 112º 5 33º to 41º 113º to 129º 6 42º to 53º 130º to 179º 7 54º to 89º 180º 8 90º Table 10-3 Sky Cover Amount Evaluation Nighttime Amount Evaluation. More time is required for night evaluation of the sky. Allow approximately 10 to 15 minutes in the darkness for eyes to adjust from lighted workspaces to darkness before evaluating the celestial dome. Additionally, a longer observation period will allow the observer to see clouds pass overhead and obscure the stars. Consider the sky to be clear if the stars are plainly visible over the majority of the sky and no cloud or obscuring phenomena is observed. When the stars are dimmed, the dimming may be evidence of the presence of cloud or obscuring phenomena and will be of assistance in determining the amount of sky covered. 10-6

109 Determining Sky Cover Classification. Select the appropriate sky cover contraction or combination of contractions to be reported after evaluating the following: Step 1. Estimate (to the nearest eighth) the amount of sky covered by the lowest layer present. If this layer is a surface-based obscuring phenomenon, determine only the amount of sky that is hidden. Transparent surface-based atmospheric phenomena do not constitute sky cover. Step 2. Determine if additional layers of clouds and/or obscuring phenomena aloft are present above the lowest layer. Using the summation principle, estimate the eighths of sky covered by each of these layers in combination with the lower layers. Do not add to the total coverage amounts visible through transparencies in lower layers, except those amounts of upper layers visible through transparent surface-based atmospheric phenomena. Step 3. Repeat the evaluation in step 2 for each additional layer present in ascending order of height. Estimate the summation (in eighths) of sky covered by each layer, in combination with all lower layers. Step 4. Assign a reportable value to each layer as given in Table SKY COVER CLASSIFICATION CLASSIFICATION MEANING SUMMATION AMOUNT OF LAYER SKC or CLR (1) Clear - Sky without clouds or 0/8 obscurations FEW (2) A few clouds are present >0/8 to and including 2/8 SCT (2) Scattered clouds are present 3/8 to and including 4/8 BKN (2) Broken - More than half, but not all of the 5/8 to and including 7/8 sky is covered OVC Overcast - the sky is covered by clouds 8/8 VV Vertical Visibility - the sky is totally obscured by obscuring phenomena 8/8 (1) CLR is used by ASOS when no clouds below 12,000 feet are detected; SKC is used when a manual observation determined there are no clouds present. (2) A partial obscuration could make up part or all of these classifications Variable Classification Conditions Table 10-4 Sky Cover Classification Variable Sky Condition. If while observing the sky, the classification varies between one or more reportable values, this condition shall be reported (i.e. BKN V SCT) Non-Uniform Sky Condition. Observers shall be alert to variations in sky condition that are not reflected in the sky cover reported in the body of the report. When non-uniform sky conditions are observed; for example a significantly lower ceiling in a particular direction from the station, it shall be described in the report as, CIG LWR N This would indicate that the ceiling is lower to the north. 10-7

110 Determination of Ceiling. By definition, the ceiling is the height above the earth s surface ascribed to the lowest broken or overcast non surface-based layer; OR the vertical visibility into a surface based obscuring phenomena that totally hides the sky. Therefore, by completing the above evaluations, the observer has determined the ceiling. Although surface-based partial obscurations can not be considered a ceiling, the amount of the partial obscuration is used in the summation principle, e.g. 3/8 of fog obscures the sky and 3/8 of status exists at 500 feet; therefore the ceiling is 500 feet. Since the ceiling is of special significance to flight operations, often determining whether they can be accomplished, the observer must be especially attentive to determining the height of this cloud layer Variable Ceiling. When the height of a ceiling layer increases and decreases rapidly during the period of evaluation by the amounts given in Table 10-5 (Criteria for Variable Ceiling), and the ceiling height is below 3,000 feet, it shall be considered variable and the ascribed height shall be the average of all the values. A remark shall be included in the observation giving the range of variability. Variable ceilings at or above 3,000 feet may be reported as variable only if considered operationally significant. CEILING Equal to or less than 1,000 feet Greater than 1,000, and equal to less than 2,000 feet Greater than 2,000 but less than 3,000 feet. VARIATION Equal to or greater than 200 feet Equal to or greater than 400 feet Greater than 500 feet Table 10-5 Criteria for Variable Ceiling Significant Clouds. Observers shall be alert for the occurrence of cumulonimbus, towering cumulus, altocumulus castellanus, standing lenticular, or rotor clouds and report them whenever they occur. The evaluation of these clouds shall include the identification of the cloud, and (insofar as known) the direction and distance from the station and, for cumulonimbus clouds, the direction of movement Examples of Sky Condition Evaluation. Sky cover is the most subjective portion of the observation. Some examples of sky cover evaluation and summation are given in Table

111 OBSERVED SKY COVER SUMMATION CLASSIFI OVERALL SKY EVALUATION CATION Example 1 3/8 obscured by fog 3/8 SCT 2/8 clouds observed 1,000 ft 5/8 BKN 2/8 clouds observed at 8,000 ft 7/8 BKN SCT000 BKN010 BKN080 Example 2 Less than 1/8 clouds observed 0/8 FEW at 500 ft 1/8 clouds observed at 2,000 ft 1/8 FEW 4/8 clouds observed at 5,000 ft 5/8 BKN Less than 1/8 clouds observed at 12,000 ft 5/8 (could be 6/8) BKN Example 3 1/8 layer of smoke aloft at 1/8 FEW 1,000 ft FEW005 FEW020 BKN050 BKN120 5/8 clouds observed at 8,000 ft 6/8 BKN 2/8 clouds observed at 25,000 8/8 OVC FEW010 BKN080 OVC250 ft Example 4 Sky totally hidden by snow, vertical visibility 300 ft 8/8 VV VV003 Example 5 6/8 of the sky hidden by fog 6/8 BKN 2/8 clouds observed at 2,000 ft 8/8 OVC BKN000 OVC020 Example 6 2/8 clouds observed at 1,500 ft 2/8 FEW 3/8 clouds observed at 3,000 ft 5/8 BKN 3/8 clouds observed at 7,000 ft 8/8 OVC FEW015 BKN030 OVC070 Pilot reports layer at 7,000 ft is SCT Table 10-6 Sky Cover Evaluation Examples Sky cover is still evaluated as OVC at 7000 ft. Even though pilot reports layer as SCT, the observer must report what he sees from the ground, and in this case the layer at 7,000 ft when considered with the lower layers covering the sky causes the sky to be OVC at 7,

112 CHAPTER 11 - TEMPERATURE AND DEW POINT OBSERVATIONS This chapter describes procedures for observing and determining the air temperature and dew point temperature for a surface report. The temperature data obtained using the instruments in this chapter are in terms of the Celsius scale; however, temperature may be observed and reported in either Celsius or Fahrenheit since some instruments are marked in only one scale and some reports require one specific scale Definitions Ambient Temperature. A measure of the average kinetic energy of the molecules of the surrounding air not affected by radiant heat from objects or other outside influences such as exhausts. The reportable air temperature Dry-Bulb Temperature. The ambient temperature as indicated by a hygrothermometer and regular thermometer or the dry bulb of a psychrometer Wet-Bulb Temperature. The temperature an air parcel would have, if cooled to saturation at constant pressure by evaporation of water into it. The temperature indicated by the wet bulb (a wet wick covering the bulb) of a psychrometer. It differs from the dry-bulb temperature by an amount dependent on the moisture content of the air and, therefore, is generally the same as, or lower than the dry-bulb temperature Wet Bulb Depression. The mathematical difference between the wet and dry bulb readings Dew Point. The temperature to which a given parcel of air must be cooled at constant pressure and constant water vapor content in order for saturation to occur. Can be calculated from wet and dry bulb readings Relative Humidity. The ratio, expressed as a percentage, of the actual vapor pressure of the air to the saturation vapor pressure, usually calculated from temperature and dew point Heat Index (HI). An index used to provide the general public with advisories regarding the risks of hot weather. It provides an indication of the temperature a human feels when heat and humidity are combined. The heat index is based upon shady, light wind conditions. The index provides an indication of the danger to human health of high temperatures and high humidity. Exposure to direct sunlight can increase the HI by up to 15 F, while a breeze reduces the heat index The Wet Bulb Globe Temperature (WBGT). Another index used primarily by the military to provide a sense of the risks associated with hot weather and takes into account the additional heat stress dues to radiation Wind Chill Index. The portion of the cooling of a human body caused by air motion. Air motion accelerates the rate of heat transfer from a human body to the 11-1

113 surrounding atmosphere, especially when temperatures are below about 7C (45F). The index provides an indication of the potential danger from the combination of wind and cold temperatures Psychrometer. An instrument used for measuring the ambient air temperature and the water vapor content of the air. It consists of two ordinary glass thermometers. The bulb end of one thermometer is covered with a clean muslin wick that is saturated with water prior to an observation (the wet-bulb). When the bulbs are properly ventilated, they indicate the wet and dry-bulb temperatures of the atmosphere. Ventilation is achieved by whirling a hand psychrometer, or by a small fan on an electric psychrometer until the lowest wet-bulb temperature has been obtained Hygrothermometer. An electronic instrument, either portable or fixed, that senses the ambient temperature and water vapor content of the atmosphere. The instrument calculates dew point and a variety of other parameters including relative humidity, heat index, etc Psychrometric Calculator or Computer. A circular slide rule used to manually calculate dew point and relative humidity from known values of dry and wet-bulb temperature and the station pressure. It also has conversion scales for Celsius and Fahrenheit Psychrometric Tables. Tables prepared from the psychrometric formula and used to determine dewpoint and relative humidity from known values of dry and wetbulb temperatures Evaluating Temperature and Dew Point. The method of obtaining temperature and dew point and other derived parameters varies according to the system in use at the station. The data may be read directly from digital or dial readouts, or calculated from other measured values Units of Measure. The United States uses both the Fahrenheit and Celsius temperature scales for weather observations. Figures 11-3 and 11-4 provide conversion tables. Air temperature and dewpoint data are evaluated to the nearest tenth of a degree whether reading from an instrument calibrated in Fahrenheit or one calibrated in Celsius Observation Period. When using manual instruments (not the ASOS) the observation period should be at least 10 minutes to allow for acclimatization of the instrument; and if using a psychrometer, equilibrium of the wet bulb reading Priority of Instruments. When the fixed automatic sensing system (e.g. ASOS or equivalent) is available and functioning within operational limits, obtain air, dew point and other derived parameters by direct reading of OID. Otherwise, obtain the base readings from a portable hygrothermometer or psychrometer and calculate other parameters using a psychrometric calculator or table Use of Psychrometers. Obtain dry-bulb and wet-bulb temperature values by following the instructions below; detailed use is provided in operating handbooks. 11-2

114 Exposure. Prior to actual use for temperature measurements, the psychrometer must be exposed to the outside free air (in a shaded location) long enough to allow the instrument to reach thermal equilibrium (at least 5 minutes). When taking the reading, avoid direct sunlight, and hold the instrument away from your body, as well as any surface that might radiate heat energy. Face into the wind when using any psychrometer Preparation. Moisten the Wet-Bulb wick prior to each use. Water used to moisten the wet-bulb thermometer wick must be free of mineral matter to allow proper evaporation. Use distilled water, rainwater, or melted snow. Store the water in a covered container and replace it as often as necessary (usually once a week). Change the wick as often as necessary to ensure a clean wick is used. When the wet-bulb temperature is above 37 F (3 C), moisten the wick just before ventilating (even if the humidity is high and the wick appears wet) Preparation in Temperatures of 37 F (3 C) or below. Use room temperature water to completely melt any accumulation of ice on the wick. Moisten the wick thoroughly (at least 15 minutes before ventilation) to permit the latent heat of fusion (released when water freezes) to dissipate before ventilation begins. Do not allow excess water to remain on the wick since a thin ice coating is necessary for accurate data. If the wick is not frozen at wet-bulb temperatures below 32 F (O C), touch the wick with clean ice, snow, or other cold objects to induce freezing. If you are unable to induce freezing, use the low temperature range of the psychrometric calculator for computation Preparation in Dry Climates. Use pre-cooled water whenever practical in areas where the temperature is high and the relative humidity is low. Moisten the wick thoroughly several minutes before and again at the time of ventilation. This helps reduce the temperature and prevents the wick from drying out during ventilation. When this procedure is not completely effective, keep the wick extended into an open container of water between observations Cold Weather Preparation. If the psychrometer is kept outside, when dew or frost are expected, check the dry-bulb thermometer 10 to 15 minutes prior to ventilation. Remove any collection on the thermometer with a soft clean cloth and allow sufficient time for the dissipation of extraneous heat before ventilation. When precipitation is occurring, the dry-bulb temperature must be obtained before any moisture falls on the dry bulb. If there is moisture on the thermometer, wipe it dry with a soft clean cloth and shield the thermometer from the precipitation as long as necessary. This permits dissipation of any extraneous heat before reading the temperature Ventilation and Reading of Psychrometers. To ensure proper ventilation of the sling psychrometer, the air should pass over the psychrometer bulbs at a minimum of 15 feet per second. If using an electric psychrometer, turn on the fan. If using a sling psychrometer, swing the instrument so it revolves at two revolutions per second. Hold the instrument to your front and waist high. Keep the psychrometer in the shade of your body as much as practical. Begin by ventilating the psychrometer for about 15 seconds. 11-3

115 Read the wet-bulb thermometer, making a note of the reading. Ventilate for another 10 seconds and again note the wet-bulb reading. Continue this process until two consecutive readings show no further decrease. This is the wet-bulb temperature. Read both wet and dry-bulb temperatures to the nearest 0.1 F. If the wet-bulb temperature rises between successive readings, remoisten the wick and ventilate again. In reading the thermometers, make sure the line of sight from the eye to the top of the liquid column is at such an angle (approximately 90 degrees) as to minimize the error of parallax. Obtain readings with reference to the middle of the degree markings Use of Portable Hygrothermometers. (Kestrels et. al.) These instruments come in a variety of configurations with electronic sensors. The instruments each have their own operator s manual. Readings are taken directly from the electronic display for temperature, dew point, wet bulb and relative humidity. Many calculate additional parameters such as wind chill and heat index. Temperature is read to the nearest.1 degree. Exposure of hygrothermometers is similar to that of psychrometers. Prior to use, expose the instrument to the outside free air (in a shaded location) long enough to allow it to reach thermal equilibrium (at least 5 minutes). When taking the reading, avoid direct sunlight, and hold instrument away from your body, as well as any surface that might radiate heat energy. Face into the wind when using any hygrothermometer. Monitor readings until stabilized Dewpoint Equal to, or Exceeding Air Temperature. If the instrument in use is functioning within operational limits, and fog is present and the wick of the wet-bulb is not frozen, assume the wet-bulb and dew point temperatures with respect to water to be the same as the dry-bulb temperature (the relative humidity will be 100%). If ice fog is present or the wet-bulb wick is frozen, assume the wet-bulb and dew point temperatures with respect to ice to be the same as the dry-bulb and convert them to their water equivalent using the psychrometric calculator (the relative humidity will be 100%) Use of the Psychrometric Calculator or Computer. This instrument is essentially a circular slide rule designed to compute temperature/moisture parameters. It has sections designed to compute dew point and relative humidity, as well as convert temperature between Celsius and Fahrenheit. One side is for calculations above 0 Deg. C and the other is for calculations below 0 Deg. C. On the low-temperature face of the calculator, equivalent values of dew point appear opposite each other on the DP (or T w, DP) and T i scale; e.g., a dew point of 20 F (-6.7 C) with respect to ice is equivalent to 18.5 F (-7.5 C) with respect to water. For dew point calculations, it is important to use the correct pressure scale. This determination is made based on the current station pressure. Stations at or near sea level use the 30-inch scale most of the time. The wet bulb temperature is aligned with the computer index. The dew point is read opposite the wet bulb depression from the appropriate pressure scale. Detailed instructions for its use are printed on the face of the calculator. 11-4

116 11.3 Heat Index and Wind Chill. Although not a part of the official observation, these derived values are often required. Both are indicators of the combined effects of weather elements on the human body and can be a critical element of safety Determining Heat Index Values. The formula for Heat Index is somewhat complicated and not often used in the routine determination of the index: [(HI = *T *RH *T*RH *T *RH *T 2 *RH *T*RH *(T*RH) 2 ]. T equals temperature in degrees Fahrenheit and RH is humidity in whole numbers. A table such as that in Table 11-1 is most often used to determine heat index. RELATIVE HUMIDITY % TEMPERATURE Degrees Fahrenheit HEAT INDEX Affects on the human body 130 or above Extreme Danger Heat stroke highly likely. Danger Sunstroke, heat 105 to 130 cramps, and heat exhaustion likely; Heat stroke likely with prolonged exposure 90 to 105 Extreme Caution Sunstroke, heat cramps, and heat exhaustion possible Table 11-1 Heat Index Table 11-5

117 Determining Wet Bulb Globe Temperature. This index is determined using three different thermometers: (1) A standard dry bulb thermometer (Dry Bulb Temperature; DB); (2) a standard Wet Bulb Temperature; WB; and (3) a standard dry bulb thermometer whose bulb is inserted into a large (6 inch) black ball, to allow measurement of the effects of sunshine and other radiant heat (Black Globe Temperature; GT). These three temperatures are integrated as follows: WBGT = 0.7 WB GT DB Determining Wind Chill Values. The wind chill index was developed to describe the relative discomfort/danger resulting from the combination of wind and temperature. The formula, as modified and adopted by a special OFCM committee in November of 2001, uses wind speed calculated at the average height (5 feet) of the human body's face instead of 33 feet (the standard anemometer height); is based on a human face model; incorporates modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days); lowers the calm wind threshold to 3 mph; uses a consistent standard for skin tissue resistance; and assumes the worst case scenario for solar radiation (clear night sky). Bright sunshine may increase the wind chill temperature (i.e. lessen the actual chill) by 10 to 18 degrees F. The formula [Wind Chill (ºF) = T (V^0.16) T(V^0.16)]; where: T = Air Temperature (F), V = Wind Speed (mph) and ^ = raised to a power (exponential). Again most personnel rely on a table such as the one in Table Table 11-2 Wind Chill Table 11-6