Atmosphere Properties and Instruments. Outline. AT351 Lab 2 January 30th, 2008

Transcription

1 Atmosphere Properties and Instruments AT351 Lab 2 January 30th, 2008 Outline 1. Atmospheric Variables and How We Measure Them 2. Composition of the Atmosphere 3. How to Represent Weather Data Visually 1

2 Atmospheric Properties Temperature Pressure Moisture (i.e. Water Vapor) Density Wind (i.e Velocity) SI Measurements International System of Units Length [m] Time [s] Mass [kg] Temperature [K] Pressure [Pa] Energy [J] Power [J s -1 ] 2

3 Temperature A measure of the average kinetic energy due to the random movement of atoms Faster motion of molecules = higher temperature No motion = absolute zero Measuring Temperature Three main scales Kelvin Celsius Fahrenheit Conversions F = 9 *C C = (F! 32) * 5 9 K = C

4 Measuring Temperature Thermometers Liquid in Glass Measures expansion/contraction of alcohol or mercury Thermocouple Converts thermal potential difference into electrical potential difference between two ends of a conductor Thermistor Measures changes in the electrical resistance of a conductor Pressure A force exerted per unit area. In the atmosphere it is a measure of the weight of the air above you. Units Pa [N m -2 ] 1 mb = 100 Pa in Hg atm Standard Pressure (Sea Level) mb in Hg 1 atm Typical Values Fort Collins 850 mb From Units Sheet: inhg Pressure = Force/Area atm Force = Mass*Acceleration Long s Peak: 600 mb Mt. Everest: 300 mb AIR HAS MASS???? 4

5 Measuring Pressure Mercury Barometer Aneroid Barometer Barograph Water Vapor Specific Humidity (q) The actual amount of water vapor in the air. Vapor compared to ALL air [g kg -1 ]. Mixing Ratio (r) The actual amount of water vapor in the air. Vapor compared to DRY air [g kg -1 ]. Relative Humidity (RH) Ratio of the amount of water vapor that exists to the amount of water vapor required for saturation. Expressed as a percentage. (T and P dependant) Dew point Temperature (T d ) The temperature at which the atmosphere will become saturated (100% RH) 5

6 Hygrometers Measuring Water Vapor Electronic Sling Psychrometer Chilled Mirror Hair Wind Wind direction is named for where the wind is coming from. Expressed in either cardinal directions (N, S, E, W, NW, SE, etc.) or degrees from north Mathematical 180 Meteorological 6

7 Measured in: Measuring Wind Miles per hour [mph]: what you re used to Nautical Miles per hour knots or kts]: What we use on weather maps 1 mph = kts Meters per second [m s -1 ]: What we use for calculations 1 mph = m s -1 The Atmosphere 7

8 Atmospheric Properties Carbon Dioxide, CO 2 8

9 Methane, CH 4 Ozone, O 3 In the stratosphere, ozone serves as a heat source, as it absorbs ultraviolet radiation. In the troposphere, ozone is a pollutant. 9

10 Atmospheric Properties 6.5 o C/km lapse rate in the troposphere Atmospheric Properties The air is a compressible fluid. Water is relatively incompressible. dp dz =!"g 10

11 Wait a Minute If hot air rises, why is it cold at the top of mountains? And why does the tropopause cool with height? The Ideal Gas Law Relates the temperature, pressure, and volume of an ideal gas R* has units of [J mol -1 K -1 ] R= J kg -1 K -1 The atmosphere is close to being an ideal gas. Increasing pressure means increasing temperature. PV = nr * T n = m MW n V =! MW P =! MW R* T R = R* MW = J! K "1! kmol "1 = J! kg "1! K " kg! kmol "1 P =!RT 11

12 Temperature ( o F) Station Plots G23 Wind speed, direction, and peak gust Station Pressure Visibility (miles) Current Weather 9 45 FNL hour pressure change Dewpoint ( o F) Sky Cover Station ID 3 hour precipitation Other Station Plots Older Version 12

13 Reading Station Plots Pressure 000 to 499: mb to mb 500 to 999: Pressure Tendency mb to mb In tenths of a mb change in 3 hours E.g., -32 is a drop of 3.2 mb in 3 hrs Temperature in o F Reading Station Plots 13

14 Reading Station Plots 14

15 Upper Air Isopleths A line that connects equal values of some quantity. (iso = same in Greek) Examples Isotherm: line of constant temperature (5 or 10 F intervals) Isodrosotherm: line of constant dew point (5 or 10 F) Isobar: line of constant pressure (4 mb) Isotach: line of constant wind speed(5 knots) Contouring Drawing isopleths on weather maps Lines closer together mean the value is changing more rapidly over a short distance (cold front) 15

16 Rules for Contouring 1) Isopleths should never cross. 2) Isopleths should not branch or fork. 3) Do not draw isopleths where there is no data. 4) Label at both ends, and color code the isopleths. 5) Isopleths can form closed loops. 6) Eliminate what appears to be incorrect data. 7) Start with locating highest and lowest values. 8) Interpolate between given values. 9) Lines should be smooth. 16

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18 Websites Surface observations and analyses Upper air maps and observations