Fire Weather: Winds

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 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

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

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?

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

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

• 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

• “P” = polar• “T” = tropical• “m” = maritime• “c” = continental

Polar – coldnessTropics – heat Oceans – moistureContinents - 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?– 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• 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)