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WEATHER

WEATHER. Fire Weather. *Click on image to play video. CPBM Objectives. Describe the various types of wind and how wind influences fire behavior. Understand the temp/humidity relationship. Define stability, inversion, mixing height, dispersion index and transport wind speed.

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WEATHER

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  1. WEATHER

  2. Fire Weather *Click on image to play video

  3. CPBM Objectives • Describe the various types of wind and how wind influences fire behavior. • Understand the temp/humidity relationship. • Define stability, inversion, mixing height, dispersion index and transport wind speed. • List the methods by which heat is transferred, and how weather affects these. • Explain the sea breeze process and the possible effects on Rx fires.

  4. CPBM Objectives • Describe the passage of a typical cold front in Florida, particularly as it relates to wind and temperature. • List the influences of thunderstorms on fire behavior. • Understand the differences and usages of the daily planning, spot, red flag event and other weather forecasts. • Use a Belt Weather Kit to take a representative weather observation. • Discuss the steps in obtaining a spot weather forecast.

  5. Temperature • The hotness/coldness of a substance • Reflects average molecular activity (thermal energy) • Fahrenheit • Celsius • Kelvin (0 = no molecular activity)

  6. What influences air temperature? • Time of day • Season • Elevation • Topography • Latitude • Weather systems • Bodies of water Why does air temperature matter in wildland fires?

  7. Heat vs. Temperature • Heat Energy • Can be converted to other energy forms • Can be transferred from one substance to another • Temperature • Determines direction of net heat transfer (hot to cold)

  8. Heat Energy applied = rise in temperature (if NO physical structural change occurs)

  9. Temperature change depends on molecular structure (“Heat Capacity”) Examples of Specific Heats (relative to water) • Water: 1.0 • Forests: 0.45 - 0.65 • Dry Air: 0.24 • Dry Soil/Rock: 0.20 • Gasoline: 0.50

  10. Solar heatingeffect of latitude

  11. SUMMER WINTER

  12. Atmospheric Reaction • Unequal heating of the earth. • Energy absorbed and energy released are not in balance at any one time and place.

  13. The attempt to regain balance causes atmospheric disturbance • THIS IS WEATHER!

  14. Atmospheric Layers

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

  16. 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.

  17. General Winds *Click on image to play video

  18. Local Winds *Click on image to play video

  19. 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

  20. 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.

  21. Local WindsSea/Land Breeze

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

  23. Seasonal Weather *Click on image to play video

  24. Winter Weather in FLCold Fronts *Click on image to play video

  25. Cold Front Passage • Wind shifts as the front passes • clockwise SW to W/NW • Winds increase in speed • Air becomes drier Implications for fire? -Rx fire -wildfire

  26. 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%

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

  28. Thermograph Temperature and RH Relationship

  29. RH and Fire • Fuel Moisture -fine versus large fuels? • Why does this matter • wildland fire suppression • Rx burning Small changes in RH that aren’t noticed can have significant impacts • Monitoring (belt weather kit) is crucial!!

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

  31. 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 Keetch/Byram Drought Index (KBDI: 0 to 800)

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

  33. AtmosphericStability • Depends on the vertical temperature distribution of the atmosphere • Unstable air encourages vertical motion

  34. AtmosphericStability • Environmental Lapse Rate • Change in temperature with height within the air column • Varies widely depending on conditions (average 3.5˚F/1000ft) • Determines atmospheric stability • Adiabatic Lapse Rate • Change in temperature solely due to change in air pressure (when a parcel of air is physically moved) • Dry (unsaturated): 5.5˚F/1000ft • Moist (saturated): 2.5˚F/1000ft is typical

  35. Initial Lifting Forces

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

  37. 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

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

  39. Stable AtmosphereInversion

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

  41. 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

  42. Unstable Atmosphere Encouraging upward vertical motion

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

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

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

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

  47. Atmospheric Stability • Mixing Height (500m minimum!) • Atmospheric Dispersion Index (ADI) 0 40 70 Poor to Fair May be too stagnant for Rx Burning (smoke issues) Good Preferred range for Rx Burning Very Good May be too hazardous for Rx Burning (control issues) • LVORI (low visibility occurrence risk) • Calculated from ADI and relative humidity • Associated with risk of vehicle accidents • Range 0-10

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