1. Fire Weather: Winds

2. WINDS: general rules
All winds blow in response to pressure differences: from high to low pressure
What factors create pressure differences?
Temperature differences: Heating or cooling
Pressure cells and fronts

3. Surface weather map
Lines (isobars) through points of equal sea-level pressure
Outline areas of high and low pressure
Low-pressure centers: “Lows”
Line of low pressure: “Trough”
High-pressure area: “High”
Lines of high pressure: “Ridge”

4. High and Low Pressure Cells
Winds blow inward, lifting (convection)
Counterclockwise direction
Cooling, increased RH
Can produce condensation, clouds, rain
Winds blow outward, descending (subsidence)
Clockwise direction
Warming, may be very dry air
Few clouds, little or no rain

5. General Winds Local Winds Gradient winds
Undisturbed by the topography of the earth
Local Winds
= small-scale winds caused by local
temperature (and pressure) differences
Affected by the topography and surface
of the Earth

6. Local Winds: General Rules
Warm air – rises or sinks?
Cool air – rises or sinks?
Cool, moist air = high or low pressure?
Warm, dry air = high or low pressure?
Heating of land vs. water – which heats faster?
Air temperature change between night and day?

7. TTYP
You will receive a picture of one of the following types of local winds. Develop a definition (in your own words) of the processes that cause that wind to blow. How do you think that particular wind will affect fire behavior?
Sea breeze – land breeze
Slope wind
Valley wind
Foehn wind
Turbulence
Thunderstorm drafts

8. Sea Breeze – Land Breeze

9. Sea Breeze – Land Breeze
Heating of land surface greater than water (water – high conductance/transparency)
Low pressure over land
Cool, moist air (high pressure) flows from sea to land
Nighttime: Land surface cools more quickly than water surface – reverse circulation
Wind speeds greater for sea breeze than for land breeze

10. Slope Winds

11. Slope Wind Upslope winds – day Downslope winds – night
Heating of surface air
Downslope winds – night
Cooling of surface air
Cooled dense air falls (gravity)
Usually not as strong as upslope winds

12. Valley Winds

13. Valley Winds Air in valleys becomes warmer than adjacent plains
Creates pressure gradient (lower pressure in valley) = upvalley winds
Begin late morning or early afternoon (most air warmed)
Downvalley winds = early night – cool, heavy air sinks

14. Foehn Winds
Chinook – moist air rises up mountain, cools (dew point  condensation), releases moisture, cold dense air descends, compressed, dries (Rockies)
Santa ana – fall, ocean warmer (low pressure) compared to mountain air (cooler, high pressure)

15. Foehn (“fern”) Winds Mountain systems Warm and dry downflowing wind
Creates critical fire-weather situations
Three forces:
Gravity – cool, heavy air descends down leeward side
Compression – descending air on leeward side = warm, dry
Pressure gradient – different pressures on either side of the mountain moves lower pressure air away from lee-side (replaced by warm foehn)

16. Turbulence

17. Turbulence Mechanical turbulence – surface friction
Mountain waves
Eddies (lee side of trees, canyons)
Tree canopies
Thermal turbulence - intense local heating & convection
Whirlwinds, dust devils, firewhirls

18. Thunderstorm drafts

19. Fire Weather: Air Masses and Fronts

20. Air Mass Body of air having uniform characteristics
Temperature
Humidity
Acquire characteristics of source region

21. Classification of air masses
“P” = polar
“T” = tropical
“m” = maritime
“c” = continental
Polar – coldness
Tropics – heat
Oceans – moisture
Continents - dryness

23. Fronts What is a “front”? Two air masses meet
Different densities (don’t mix)
Surface of contact = “front”
What happens when warm and cold air meet? What are the possible effects on fire behavior?
Gusty winds
Turbulence
Lightning & storms
Dry air

24. Fronts formed in troughs of low pressure

25. Cold front Cold air replaces warm air Faster moving, steeper slope
High temperature gradient

26. Cold front: wind shifts
Winds ahead of front S/SW
Winds shift 45 – 180 degrees – from W NW N
Wind change sharp and distinct
Winds often gusty, unstable air
Squall lines – line of showers & T-storms

27. Effect of cold fronts on fire?
Severe fire weather
Especially dry cold fronts

28. Warm Front warm air replaces cold air
flat surface = clouds and rain spread out

29. Warm front: Wind shifts
Winds ahead of front SE/S  shift to SW
Change in wind direction degrees – clockwise direction (N hemisphere)
Steady winds before and after frontal passage (air next to ground stable)

30. Effect of warm fronts on fire?
Fire weather often positive
Moist air and steady rain

31. Stationary Front Forces on 2 air masses similar – little movement
Surface winds blow parallel to front, in opposite directions
Weather: similar to warm front, less intense
May quickly change to moving fronts

32. Occluded Front
Cold front moves faster than warm front – overtakes it – warm air forced aloft between 2 cold fronts

33. Occluded fronts: wind shifts
Wind shift usually > 90 degrees
Winds shift from S to W or NW
Weather: cloudiness and rain (severity depends on temperatures of fronts)