fire weather: winds
DESCRIPTION
Fire Weather: Winds. 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. Surface weather map. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/1.jpg)
Fire Weather:
Winds
![Page 2: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/2.jpg)
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
![Page 3: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/3.jpg)
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”
![Page 4: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/4.jpg)
High and Low Pressure Cells
Winds blow outward, descending (subsidence)
Clockwise direction
Warming, may be very dry air
Few clouds, little or no rain
Winds blow inward, lifting (convection)
Counterclockwise direction
Cooling, increased RH
Can produce condensation, clouds, rain
![Page 5: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/5.jpg)
General Winds
• Gradient winds – Undisturbed by the topography of the earth
• = small-scale winds caused by local temperature (and pressure) differences • Affected by the topography and surface of the Earth
Local Winds
![Page 6: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/6.jpg)
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?
![Page 7: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/7.jpg)
TTYPYou 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
![Page 8: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/8.jpg)
Sea Breeze – Land Breeze
![Page 9: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/9.jpg)
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
![Page 10: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/10.jpg)
Slope Winds
![Page 11: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/11.jpg)
Slope Wind
• Upslope winds – day– Heating of surface air
• Downslope winds – night– Cooling of surface air – Cooled dense air falls (gravity)– Usually not as strong as upslope winds
![Page 12: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/12.jpg)
Valley Winds
![Page 13: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/13.jpg)
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
![Page 14: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/14.jpg)
Foehn Winds
![Page 15: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/15.jpg)
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)
![Page 16: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/16.jpg)
Turbulence
![Page 17: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/17.jpg)
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
![Page 18: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/18.jpg)
Thunderstorm drafts
![Page 19: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/19.jpg)
Fire Weather:
Air Masses and Fronts
![Page 20: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/20.jpg)
Air Mass
• Body of air having uniform characteristics– Temperature– Humidity
• Acquire characteristics of source region
![Page 21: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/21.jpg)
Classification of air masses
• “P” = polar• “T” = tropical• “m” = maritime• “c” = continental
Polar – coldnessTropics – heat Oceans – moistureContinents - dryness
![Page 22: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/22.jpg)
![Page 23: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/23.jpg)
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
![Page 24: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/24.jpg)
Fronts formed in troughs of low pressure
![Page 25: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/25.jpg)
Cold front• Cold air replaces warm air• Faster moving, steeper slope• High temperature gradient
![Page 26: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/26.jpg)
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
![Page 27: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/27.jpg)
Effect of cold fronts on fire?
• Severe fire weather • Especially dry cold fronts
![Page 28: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/28.jpg)
Warm Front• warm air replaces cold air• flat surface = clouds and rain spread out
![Page 29: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/29.jpg)
Warm front: Wind shifts• Winds ahead of front SE/S shift to SW• Change in wind direction 45-90 degrees –
clockwise direction (N hemisphere)• Steady winds before and after frontal
passage (air next to ground stable)
![Page 30: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/30.jpg)
Effect of warm fronts on fire?
• Fire weather often positive• Moist air and steady rain
![Page 31: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/31.jpg)
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
![Page 32: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/32.jpg)
Occluded Front• Cold front moves faster than warm front –
overtakes it – warm air forced aloft between 2 cold fronts
![Page 33: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/33.jpg)
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)
![Page 34: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/34.jpg)
![Page 35: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/35.jpg)
![Page 36: Fire Weather: Winds](https://reader036.vdocument.in/reader036/viewer/2022062520/56815c59550346895dca5cbd/html5/thumbnails/36.jpg)