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Lesson 2C - Weather

Lesson 2C - Weather. 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.

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Lesson 2C - Weather

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  1. Lesson 2C - Weather

  2. 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. • Describe critical fire weather conditions. • List the different types of fire weather forecasts and outlooks available.

  3. Fire Weather *Click on image to play video

  4. Air Temperature The degree of hotness or coldness of a substance. An average of the thermal energy of a group of particles in a substance (does not depend on # of particles) In weather we refer to this as air temperature or dry bulb temperature.

  5. Temperature is measured with a thermometer calibrated either to the FAHRENHEIT scale or the CELSIUS or centigrade scale. • What influences air temperature? • Time of day • Season • Elevation • Topography • Latitude • Weather systems • Bodies of water Why does air temperature matter in wildland fires?

  6. Relative Humidity For a given air temperature, relative humidity (%) is the amount of moisture in the air divided by the amount the air could hold when saturated; usually expressed in percent. • Ranges from 1-100%...what does 100% mean?

  7. Temperature and RH Relationships 3 grams 3 grams 3 grams of H2O 50ºF 70ºF 90ºF 100% (saturated) 50% (unsaturated) 25% (unsaturated)

  8. Thermograph Temperature and RH Relationship

  9. A Temp. = 60 F RH = X B Temp. = 80 F RH = X/2 Temperature vs. RH (general) for a given volume of air Relative Humidity Temperature

  10. So what? • Think of the fire triangles…what does RH affect? • Does this differ between fine/large fuels? • Why does this matter for wildland fire suppression? • Why does this matter for prescribed burning? • Small changes in RH that aren’t noticed can have significant impacts • What should you do on a fire? • Monitoring = belt weather kit

  11. Precipitation Precipitation is liquid or solid water particles that originate in the atmosphere, and become large enough to fall to the earth’s surface.

  12. PrecipitationAmount vs. Duration • Fine Fuels • gain and losemoisture 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

  13. Atmospheric Stability The degree to which vertical motion in the atmosphere is ENHANCED or SUPPRESSED. *Click on image to play video

  14. Stable Atmosphere Stable atmosphere resists upward vertical motion *Click on image to play video

  15. Stable AtmosphereVisual Indicators • Visual Indicators • Clouds form in layers • Smoke drifts apart after limited rise • Poor visibility due to smoke or haze • Fog layers • Steady winds

  16. Stable AtmosphereVisual Indicator Examples • Clouds form in layers

  17. Stable AtmosphereVisual Indicator Examples • Smoke drifts apart after limited rise

  18. Stable AtmosphereVisual Indicator Examples • Poor visibility due to smoke or haze

  19. Stable AtmosphereVisual Indicator Examples • Fog Layers

  20. Stable AtmosphereVisual Indicator Examples • Steady Winds

  21. Stable AtmosphereInversion *Click on image to play video

  22. Stable AtmosphereInversion Types • Four Inversion Types • Nighttime (Radiation) • Frontal • Marine • Subsidence • Two most common types • Nighttime (Radiation) • Subsidence

  23. Stable AtmosphereNighttime Inversion *Click on image to play video

  24. Stable AtmosphereThermal 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

  25. Stable AtmosphereSubsidence Inversion *Click on image to play video

  26. Stable AtmosphereSubsidence 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

  27. Unstable Atmosphere Encouraging upward vertical motion

  28. Unstable AtmosphereVisual Indicators • Visual Indicators • Clouds grow vertically and smoke rises to great heights • Cumulus clouds • Good visibility • Gusty winds • Dust devils and firewhirls

  29. Unstable AtmosphereVisual Indicator Examples • Clouds grow vertically and smoke rises to great heights

  30. Unstable AtmosphereVisual Indicator Examples • Cumulus Clouds • Good Visibility

  31. Unstable AtmosphereVisual Indicator Examples • Gusty Winds • Dust devils and firewhirls *Click on image to play video

  32. Winds Horizontal movement of air *Click on image to play video

  33. WindsWind’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.

  34. General Winds *Click on image to play video

  35. Local Winds *Click on image to play video

  36. Local WindsSlope Winds • 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 facing 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

  37. Local WindsValley Winds • 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.

  38. Local WindsSea/Land Breeze

  39. Sea Breeze1998 Perry Fire Sea breeze interacting with Perry fire. Fire activity increases when sea breeze front penetrates fire.

  40. Used in Rx prescriptions • LVORI- Low Visibility Occurrence Risk Index • “Smog” index- risk of low visibility (vehicles) • Increases with RH, and low DI • From operational experience, one should be VERY CAUTIOUS ABOUT BURNING if one of the following situations occurs: • When LVORI for a nighttime forecast period is 8,9 or 10 • When ACTIVE SMOKE from stumps logs, etc. is present during the night • When there is a roadway within three miles of a burn site with open fields, logging roads, or open streams that can provide an easy transit of the smoke from the burn site to the roadway

  41. Used in Rx prescriptions • Dispersion Index • Estimates daytime and nightime stability of the atmosphere. Higher DI = greater mixing, but very high = potential fire control problems • Doubling of the DI results in a doubling of the amount of smoke the airshed can hold • RX : 41-70 • Under 30, could be OK…if small unit with little smoke, or stagnant if winds are low • Over 71: very good dispersion…but hazardous to burn

  42. Those who know how to read and heed weather! Those who think they can “weather it all”

  43. 2008 burnWinter AnnualACMF

  44. 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. • Describe critical fire weather conditions. • List the different types of fire weather forecasts and outlooks available.

  45. Critical Fire Weather *Click on image to play video

  46. Critical Fire WeatherCold Fronts *Click on image to play video

  47. Critical Fire WeatherFoehn Winds *Click on image to play video

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