Lesson 2C - Weather 2C-1-S190-EP Lesson Objectives 1. Describe the affect of temperature and relative humidity has on wildland fire behavior. 2. Describe the affect of precipitation on wildland fire behavior. 3. Describe the differences between a stable and unstable atmosphere. 4. Describe general and local winds. 5. Describe critical fire weather conditions. 6. List the different types of fire weather forecasts and outlooks available. 2C-2-S190-EP Fire Weather 2C-3-S190-EP 1
Air Temperature The degree of hotness or coldness of a substance. In weather we refer to this as air temperature or dry bulb temperature. 2C-4-S190-EP Temperature is measured with a thermometer calibrated either to the FAHRENHEIT scale or the CELSIUS or centigrade scale. 2C-5-S190-EP Relative Humidity Relative humidity is the amount of moisture in the air divided by the amount the air could hold when saturated at the same air temperature; usually expressed in percent. 2C-6-S190-EP 2
Thermograph Temperature and RH Relationship 2C-7-S190-EP Temperature and RH Relationships 3 grams 3 grams 3 grams 50ºF 100% (saturated) 70ºF 50% (unsaturated) 90ºF 25% (unsaturated) 2C-8-S190-EP 2C-9-S190-EP 3
Complete Exercise 1 2C-10-S190-EP Precipitation Precipitation is liquid or solid water particles that originate in the atmosphere, and become large enough to fall to the earth s surface. 2C-11-S190-EP Fine Fuels gain and lose moisture quickly react rapidly to precipitation Heavy Fuels gain and lose moisture slowly react slowly to precipitation Duration vs. Amount Precipitation duration has greater impact on fuel moisture than precipitation amount Precipitation Amount vs. Duration 2C-12-S190-EP 4
Atmospheric Stability The degree to which vertical motion in the atmosphere is ENHANCED or SUPPRESSED. 2C-13-S190-EP Stable Atmosphere Stable atmosphere resists upward vertical motion 2C-14-S190-EP Visual Indicators Clouds form in layers Smoke drifts apart after limited rise Poor visibility due to smoke or haze Fog layers Steady winds Stable Atmosphere Visual Indicators 2C-15-S190-EP 5
Stable Atmosphere Visual Indicator Examples Clouds form in layers 2C-16-S190-EP Stable Atmosphere Visual Indicator Examples Smoke drifts apart after limited it rise 2C-17-S190-EP Stable Atmosphere Visual Indicator Examples Poor visibility due to smoke or haze 2C-18-S190-EP 6
Stable Atmosphere Visual Indicator Examples Fog Layers 2C-19-S190-EP Stable Atmosphere Visual Indicator Examples Steady Winds 2C-20-S190-EP Stable Atmosphere Inversion 2C-21-S190-EP 7
Stable Atmosphere Inversion Types Four Inversion Types Nighttime (Radiation) Frontal Marine Subsidence Two most common types Nighttime (Radiation) Subsidence 2C-22-S190-EP Stable Atmosphere Nighttime Inversion 2C-23-S190-EP Stable Atmosphere Thermal Belt Thermal Belt Nighttime inversions in mountainous regions The warm layer typically found on the middle third of the slope Characterized by the highest minimum temperature and lowest minimum RH Fire can be very active within the thermal belt 2C-24-S190-EP 8
Stable Atmosphere Subsidence Inversion 2C-25-S190-EP Stable Atmosphere Subsidence Inversion Facts Subsidence Inversion May persist for several days May reach the surface further enhancing fire activity Typically results in clear or cloudless skies above average temperatures low relative humidity drying of fuels 2C-26-S190-EP Unstable Atmosphere Encouraging upward vertical motion 2C-27-S190-EP 9
Unstable Atmosphere Visual Indicators Visual Indicators Clouds grow vertically and smoke rises to great heights Cumulus clouds Good visibility Gusty winds Dust devils and firewhirls 2C-28-S190-EP Unstable Atmosphere Visual Indicator Examples Clouds grow vertically and smoke rises to great heights 2C-29-S190-EP Unstable Atmosphere Visual Indicator Examples Cumulus Clouds Good Visibility 2C-30-S190-EP 10
Unstable Atmosphere Visual Indicator Examples Gusty Winds Dust devils and firewhirls 2C-31-S190-EP Complete Exercise 2 2C-32-S190-EP Winds Horizontal movement of air 2C-33-S190-EP 11
Winds Wind s Effect on Wildland Fire Wind impacts the fire environment by: Increasing the supply of oxygen to the fire. Determining the direction of fire spread. Increasing the drying of the fuels. Carrying sparks and firebrands ahead of the main fire causing new spot fires. Bending flames, which results in the preheating of fuels ahead of the fire. Influencing the amount of fuel consumed by affecting the residence time of the flaming front of the fire. The stronger the wind, the shorter the residence time and the less fuel is consumed. 2C-34-S190-EP General Winds 2C-35-S190-EP Local Winds 2C-36-S190-EP 12
2C-37-S190-EP Upslope Winds A result of differential heating and convective processes along the slope Average speeds range from 3 to 8 mph Develop along east facing slope first and south and west tfacing slope by late morning Downslope Winds Air along the slope cools and sinks producing the downslope wind Average speeds range from 2 to 5 mph Develop on east facing aspects first and south and west facing aspects after sunset Local Winds Slope Winds 2C-38-S190-EP Upvalley Winds As the air warm, temperature and pressure differences within the valley or adjacent valleys result in upvalley wind flow. Strongest mid to late afternoon. Average speeds range from 10 to 15 mph. Downvalley Winds As the valley loses solar heating, the air in the valley cools. The cool air drains downvalley. Average downvalley wind speeds range from 5 to 10 mph. Typically develop a few hours after sunset. Local Winds Valley Winds 2C-39-S190-EP 13
Local Winds Sea/Land Breeze 2C-40-S190-EP Sea Breeze 1998 Perry Fire Sea breeze interacting with Perry fire. Fire activity increases when sea breeze front penetrates fire. 2C-41-S190-EP Complete Exercise 3 2C-42-S190-EP 14
Critical Fire Weather 2C-43-S190-EP Critical Fire Weather Cold Fronts 2C-44-S190-EP Critical Fire Weather Foehn Winds 2C-45-S190-EP 15
Critical Fire Weather Foehn Winds Common Foehn Winds Chinook Santa Ana Wasatch North East 2C-46-S190-EP Critical Fire Weather Thunderstorms 2C-47-S190-EP 2C-48-S190-EP 16
Critical Fire Weather Dust Devils and Firewhirls 2C-49-S190-EP Complete Exercise 4 2C-50-S190-EP Predictive Services Predictive Services Products Produce By: Agency Fire Meteorologists Intelligence Coordinator FBAN Fuels Specialists Monitors, analyzes and predicts Fire weather Fire danger Interagency fire management resource impact 2C-51-S190-EP 17
Predictive Services Predictive Services 7 Day Significant Fire Potential Produced Weekly Daily in 2006 30 Day Fire Potential Produced Monthly Season Fire Potential Produced Annually Spring Fire Season Update 2C-52-S190-EP National Weather Service National Weather Service Products produced for: Fire Weather Zones Products produced d by: Meteorologists Products geared toward: Tactical planning 2C-53-S190-EP Fire Weather Planning Forecast A narrative or tabular zone-type product Elements include: Sky/weather Temperature Relative humidity 2C-54-S190-EP 18
Spot Weather Forecast Site specific forecasts that are issued to fit time, topography and weather of a specific location. 2C-55-S190-EP Fire Weather Watches / Red Flag Warnings Issued when the combination of dry fuels and weather conditions support extreme fire behavior or ignition is occurring or expected to occur. 2C-56-S190-EP Fire Weather Watches Issued when there is a high potential for the development of a Red Flag Event. Normally issued 24 to 72 hours in advance. FIRE WEATHER WATCH NATIONAL WEATHER SERVICE MIDLAND/ODESSA TX 946 AM CST FRI JAN 27 2006...FIRE WEATHER WATCH IN EFFECT SATURDAY FOR THE GUADALUPE MOUNTAINS FOR RELATIVE HUMIDITIES OF 15 PERCENT OR LESS...SUSTAINED 20 FOOT WINDS OF 20 MPH OR GREATER AND HIGH FIRE DANGER... 2C-57-S190-EP 19
Red Flag Warnings Used to warn of an impending, or occurring Red Flag event. Red Flag Warning/Fire Weather Watch Information...RED FLAG WARNING IN EFFECT ACROSS THE SUWANNEE VALLEY REGION OF NORTH FLORIDA FROM NOON TO 5 PM EST THIS AFTERNOON FOR LONG DURATIONS OF LOW RELATIVE HUMIDITY... 2C-58-S190-EP Complete Exercise 5 2C-59-S190-EP Review Lesson Objectives 1. Describe the affect of temperature and relative humidity has on wildland fire behavior. 2. Describe the affect of precipitation on wildland fire behavior. 3. Describe the differences between a stable and unstable atmosphere. 4. Describe general and local winds. 5. Describe critical fire weather conditions. 6. List the different types of fire weather forecasts and outlooks available. 2C-60-S190-EP 20