Download Winds, Air Masses, Fronts

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
• 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”
High and Low Pressure Cells
Winds blow inward, lifting
(convection)
Winds blow outward, descending
(subsidence)
Counterclockwise direction
Clockwise direction
Cooling, increased RH
Warming, may be very dry air
Can produce condensation,
clouds, rain
Few clouds, little or no rain
General 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
Local Winds: General Rules
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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?
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
Sea Breeze – Land Breeze
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
Slope Winds
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
Valley Winds
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
Foehn Winds
Foehn (“fern”) Winds
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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)
Turbulence
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
Thunderstorm drafts
Fire Weather:
Air Masses and Fronts
Air Mass
• Body of air having uniform characteristics
– Temperature
– Humidity
• Acquire characteristics of source region
Classification of air masses
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“P” = polar
“T” = tropical
“m” = maritime
“c” = continental
Polar – coldness
Tropics – heat
Oceans – moisture
Continents - dryness
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?
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Gusty winds
Turbulence
Lightning & storms
Dry air
Fronts formed in troughs of low
pressure
Cold front
• Cold air replaces warm air
• Faster moving, steeper slope
• High temperature gradient
Cold front: wind shifts
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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
Effect of cold fronts on fire?
• Severe fire weather
• Especially dry cold fronts
Warm Front
• warm air replaces cold air
• flat surface = clouds and rain spread out
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)
Effect of warm fronts on fire?
• Fire weather often positive
• Moist air and steady rain
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
Occluded Front
• Cold front moves faster than warm front –
overtakes it – warm air forced aloft
between 2 cold fronts
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)