S290 Unit 7

7-1-S290-EPUnit 7 Wind Systems

Unit 7Wind Systems

Unit 7 Objectives

1. Define wind and wind direction.

2. Describe the effects of wind on wildland fire behavior.

3. Describe general winds around high pressure and low pressure systems.

4. Describe the cause and effect of local winds (slope/valley and land/sea breeze) on wildland fire behavior.

5. Describe typical diurnal slope and valley wind patterns, and identify these temporal patterns on a topographic map.

6. Describe critical winds and their impact on wildland fire behavior.

7. Identify three ways topography can alter wind speed and direction.

Unit 7 Objectives

8. Describe general, local, 20-foot and mid flame winds, and their relationship to each other.

9. Adjust wind speeds based on topographic location and calculate mid-flame wind speeds for the three main fuel types.

Unit 7 Objectives

WindWhat is it?

• The horizontal movement of air

• Air in motion relative to the earth’s surface

• Wind facts:– Most critical factor

affecting fire behavior– Most difficult to predict– Most variable in time and

space– Poses safety and control

problems

Winds That Influence Wildland Fire Behavior

Wind Direction

• The direction from which the wind is blowing

• North wind blows from north to south

Don’t forget!!!

Effects of Wind on Wildland Fire Behavior1. Wind carries away moisture-laden air and

thus hastens the drying of wildland fuels.

2. Once a fire ignites, wind aids combustion by increasing the supply of oxygen.

3. Wind increases fire spread by carrying heat and burning embers to new fuels (spotting).

4. Wind bends the flames closer to the unburned fuels, thus preheating the fuels ahead of the fire front.

Effects of Wind on Wildland Fire Behavior

5. The direction of the fire spread and smoke transport are determined mostly by wind direction.

6. Wind influences the amount of fuel consumed by affecting the residence time of the flaming front of the fire.

General Winds

• Winds driven by large scale high and low pressure systems.

• Winds typically found at mid and upper levels of the troposphere.

• Winds responsible for transporting weather systems around the world.

• How the earth’s general wind circulation would appear if the earth did not rotate on its axis.

General Winds

Coriolis Force

• The Coriolis Force is a result of the earth’s rotation.

• An apparent force, relative to the earth’s surface, on moving particles (air).

• Causes large-scale moving air to deflect to the right in the Northern Hemisphere.

General Wind Circulation Over the Northern Hemisphere

General wind circulation produced by horizontal and vertical changes in temperature, large scale pressure systems, and coriolis force.

General Wind Circulation

General WindJet Stream

• Strong currents of air located within the Westerlies

• Produced by pressure gradients between poles and the equator

• May be thousands of miles long, hundreds of miles wide, and thousands of feet deep

• Located near 30,000 feet

• May exceed 180 mph• Location varies from day to

day and season to season

General WindJet Stream

General WindFlow around Highs and Lows

• Troughs and ridges make up wavelike pattern

• Trough– Elongated area of low

pressure– Located on cold side or north

of jet stream

• Ridge– Elongated area of high

pressure– Located on warm side or

south of jet

Low Pressure– Pressure

decreases towards the center of the low

– The lowest pressure is located at the center of the cell

General Wind Highs and Lows

Region of LOWEST pressure Relative to its surrounding

High Pressure– Pressure

increases towards the center of the high

– The highest pressure is located at the center of the cell

7-21-S290-EP

General WindHighs and Lows vs. Ridges and Valleys

• High pressure ridge– Similar to ridges on a topo

• Low pressure trough– Similar to valleys on a topo

• Contours are similar to isobars– Contour gradient

proportional to slope steepness

– Isobar gradient proportional to wind speed

Exercise 1

Jet StreamsHigh and Lows

Local Winds• Winds found at lower

levels of the troposphere

• Induced by small-scale differences in air temperature and pressure

• Influenced by terrain; the more varied the terrain, the greater the influence

Local WindsTypes

• Diurnal Mountain Wind Systems– Winds associated with complex terrain– Slope and Valley Winds

• Land and Sea Breeze– Local wind circulation that occurs near a large

body of water

Local WindsUpslope/Upvalley

Early to Mid-Morning- 3 to 8 mph

Late Morning andAfternoon- 10 to 15 mph

7-31-S290-EPUnit 7 Wind Systems 7-31-S290-EP

Local WindsUpslope/Upvalley Transition

Local WindsDownslope/Downvalley

Late Evening and Overnight- 5 to 10 mph

Late Afternoon and

Evening- 2 to 5 mph

Local WindsSlope and Valley Wind Transition Facts

• The change from downslope-downvalley to upslope-upvalley can rapidly change fire behavior from inactive to active.

• Upslope-upvalley wind does lead to faster uphill or upvalley fire spreads.

• Downslope or downvalley winds seldom produce dangerous conditions.

• Strong downslope-downvalley winds augmented by terrain or foehn winds can result in downhill runs.

Local WindsLand and Sea Breeze

Land-Sea Circulation• Temperature and pressure

contrasts between land and sea

• Little if any temperature change over the large body of water

• Large temperature change over land from day to night

• Sea breeze during the day• Land breeze at night• Strongest in spring and

summer• Land-Sea breeze 10-20 mph

Florida Sea Breeze• Shifting winds on

either coast can cause fire control problems

• Thunderstorm activity can develop along the sea breeze front

• Fires can blow up along the sea breeze front

Local WindsSea Breeze Over Florida Example

Gulf ofMexico

Florida

AtlanticOcean

Local WindsSea Breeze

1998 Perry Fire

Slope & Valley WindExercise 2

Critical Winds• Critical Winds - winds that totally

dominate the fire environment.

• Critical Winds Include:– Frontal Winds– Foehn Winds– Thunderstorm Winds – Whirlwinds– Surfacing or Low-Level Jets – Glacier Winds

Critical WindsCold Front Winds

Fronts• Boundary between

two dissimilar air masses

• Extend from the center of low pressure

• Move at 20 to 30 mph

Cold Front• Boundary separating a

cold air mass from a warm air mass

• Can migrate west to east and north to south

Pre-Frontal Conditions• Light southeast winds 150

miles ahead of the front• Winds shifting and increasing

from the south as the front approaches

• Winds shifting southwest just ahead of the front and becoming strong

• Winds strongest along the front

• Warm air ahead of the front• Sometimes but not always a

dry air mass• Unstable air mass• Favorable burning

environment

Post-Frontal Conditions• Winds rapidly shift to the

northwest as the front passes with speeds remaining strong and gusty

• Temperatures cool rapidly

• RH increases• Fire behavior typically

decreases

Frontal Wind Speeds• Strongest winds just ahead

and behind the front where the biggest temperature/pressure gradient exists

• Light to moderate winds well ahead and behind the front as the temperature/pressure gradient decreases

• Average speeds are 15 to 30 mph

• Can be stronger with strong cold fronts

North to South Migrating Cold Fronts

• Winds ahead of the front are similar to traditional west to east moving front

• Winds behind front may be northeast or east depending on trajectory of the front

Exercise 3

Critical WindsFoehn Winds

• Strong, warm and dry winds that originate from areas of high pressure in mountainous regions

• Characteristics– Air speeds up as it flows

down the lee slopes– Rising temperatures lowering

RH on lee slopes– Typical speeds of 40 to 60

mph. Gusts in excess of 90 mph

– Extreme fire behavior is the common denominator

Foehn Winds• Chinook• Santa Ana • Mono• Wasatch• East• North

Chinook Winds• Steep pressure

gradient forms between high pressure on the windward slope and low pressure on the lee slope

Santa Ana Winds• Originates in the high deserts

of southern California. • steep pressure gradient

exists between high pressure in the Great Basin and low pressure off the coast of southern California.

• Downslope off shore flow develops.

• Can create critical fire weather situations in southern California.

Santa Ana Wind Storm• Romero fire October 10,

• Note the sudden temperature rise and RH drop when the Santa Ana winds develop

Critical WindsSanta Ana Wind Storm

Strongest downslope wind occurs at this point with the wave over the mountainridge.

The foehn wind reverses and weakens as the mountain wave movesoverhead.

The downslope foehn wind will resume but not as strongly as the wave continues to move away from the mountain ridge.

Critical WindsFoehn Winds and Mountain Waves

Two characteristicsimportant to fire

weather

• Lightning

• Indraft and downdraft winds are most important

Critical WindsThunderstorm Winds

Thunderstorm Winds• Indrafts and downdrafts can

change both direction and speed suddenly

• Result in sudden changes in rate and direction of fire as well as intensity

• Indraft speeds range from 10 to 20 mph and gusty

• Downdrafts speeds range from 25 to 35 mph with gusts over 60 mph

Gust Front• Leading edge of the

downdraft

• Boundary between two dissimilar air masses, similar to a cold front

• Most of the time, marked by a wind shift, decrease in temperature and increase in RH

Outflow Winds• Outflow wind strongest

in the direction the storm is moving

• Outflow wind weakest in the opposite direction the storm is moving

Outflow Winds• Typically spread radially

5 to 10 miles

• Topography can alter the direction of wind and speed

• Channeled through valleys, canyons, and drainages

7-58-S290-EPUnit 7 Wind Systems 7-58-S290-EP

Outflow Winds• Spread more

evenly over a flat plain

• Note the gust front located over the Texas Panhandle

Wet Thunderstorms• Storms producing greater than .10

inch• Erratic and gusty winds• Cool and moist conditions from

wet thunderstorm outflow can decrease fire activity

Dry Thunderstorms• Storm producing less than .10

inch• High cloud bases• Very strong downdrafts• Temperature and RH changes

may not be as significant as wet thunderstorms

• Impact on fire activity could be less than wet thunderstorms

Downdraft Indicators• Virga• Rain shaft • Dust cloud

Pyro-Cumulus• Cumulus development

triggered from heat rising in a convective column

• This stage should be monitored closely for further development

Pyro-Cumulonimbus• Thunderstorm generated

by heat rising in a convective column

• This stage poses a great threat to firefighters

Pyro-Cumulonimbus• Outflow onset may occur

with little or no warning• Visual indicators such as

virga or a rain shaft will likely be obscured from smoke

• A period of relative calm may be observed prior to outflow onset

• Lightning is possible along with rain

• Utilize lookouts

Glacier Winds• Local downslope winds

that impact locations adjacent to the base of glaciers

• Driven by temperature difference between the air over the ice and land

• Downslope wind of 50 mph extending 10 miles away from the glacier have been noted

• Have caught firefighters off guard

Upvalley Wind

Glacier Wind

Critical WindsGlacier Winds

Low-Level Jet• A jet stream 100 feet to

several thousand feet above ground

• Develop ahead of cold fronts or troughs

• Wind speeds of 25 to 35 mph

• Can increase lift and plume dominated fire

• Can surface and significantly increase rates of spread

Critical WindsLow-Level Jets

• Commonly form over the western Great Plains at night during the early spring and summer

• Form at night in the spring along the Sierra Nevada, and then migrate northward along the Cascades in Oregon by morning.

Oregon

Nevada

Critical WindsLow-Level Jets

Whirlwinds• Form in an unstable lower

atmosphere with relatively light winds

• May remain stationary or move with surface wind

• Vary in size from just a few feet to over 100 feet in diameter to heights of nearly 4,000 feet

Two Types Include:• Dust Devil• Firewhirl

Critical WindsWhirlwinds

Dust Devils• Occur on hot days,

under clear skies, and light winds

• Common in an area that just burned over

• On fire, they can be smoke filled

• Small-scale wind speeds greater than 50 mph

Firewhirls• Carry flames and

burning materials up into the column

• More dangerous than dust devils

• Scatters fire

• Result in spotting across control lines

• Can increase fire activity in localized areas

• Small-scale wind speeds greater than 100 mph in extreme cases

THREE WAYS TOPOGRAPHY CAN ALTER THE SPEED AND DIRECTION

OF THE WIND

• Mechanical • Turbulent • Frictional

Ways Topography Alters WindMechanical or Diverting Effects

Directional Channeling• Wind is channeled through valleys and drainages

• Airflow follows prominent terrain features

Venturi (Bernoulli) Effect• Acceleration of air through a terrain constriction,

such as a pass or gap

• Air accelerates through by the pressure gradient across the topographic constriction

Mountain Waves• Strong winds cross prominent terrain resulting in a wave• Altocumulus lenticular cloud is a good indicator of mountain waves• Air below ridge line can be turbulent• Air over lee side basins can become warm, dry and unstable

Ways Topography Alters WindTurbulent Effects

Lee-Side Turbulence “Eddying”

• Form on the lee-side of a significant terrain feature or obstruction

• May develop vertically

• May develop horizontally

• Wind speed, obstruction size and orientation, and stability determines eddy type

Strong Winds in Canyons• Eddies can form at the

confluence of tributaries

• Spur ridges can be turbulent

Thermal Turbulence• Caused by differential

heating from varying land surfaces

• Can disrupt low-level wind flow

Obstructions or Irregularities• Low-level wind flow can be

disrupted by ground level obstructions or irregularities– Trees– Cliffs– Valleys

Frictional Drag• Slows winds• Strength

dependent on varying surface roughness

Ways Topography Alters WindFrictional Drag

Wind Adjustments20-Foot Wind

20-Foot Surface Wind• Measured 20 feet

above the ground in a clearing

• Measured 20 feet above the average vegetation cover

20–Foot Surface Wind

20-FootSfc Wind=

General WindComponent + Local Wind

Component

Wind AdjustmentsCalculating 20-Foot Surface Winds

Wind AdjustmentsFor Topographic Locations

Encountering Variationsof Wind

• Across lower foothills• Intermediate hills

Basic Wind Adjustments• Are considered for hills

100’s of feet high and not mountain ranges 1000’s of feet high

MOUNTAINMOUNTAINRANGESRANGES

HILLSHILLS

Adapt forecasted or observed wind speed from one location to fit the fire’s current or expected location

Apply adjustment factor to account for variation of winds across the terrain You know the wind at

this location

What is the estimated wind for the fire location?

• Speeds on upper windward slopes are close to the speed of the ridgetop or general winds.

• Speeds on lower windward slopes are about ½ the speeds on the upper slopes.

• Winds on the lee slopes are often reduced and very turbulent.

• The smoke can show that the winds are blowing across the hill and are not just upslope on both sides.

• Ridgetop winds of 10 mph or greater would be expected to blow across the hills.

1 X8 mph

General WindLee-Side Winds• Can sometimes be

estimated• Adjustments apply

best in well mixed airflows on sunny afternoons

• Do not apply with critical winds or nighttime downslope winds

For hills with slopes 30% or less; and rounded vs. sharp ridgetops

1X 3/4X

20 mph

20 mph 15 mph5 mph10 mph

Wind Flowing over Hills andCanyons or Valleys

Wind AdjustmentsMid-Flame Wind

• Once you have a wind speed that fits the fire’s location you must adapt it to the actual conditions at mid-flame height

• Adjustment of the 20-foot wind based on flame height and sheltering is required in determining mid-flame wind speed

• Wind speeds decline closer to the ground and sheltering from trees or brush reduces the wind speed

Exercise 4

Unit 7 Objectives

1. Define wind and wind direction.

2. Describe the effects of wind on wildland fire behavior.

3. Describe general winds around high pressure and low pressure systems.

4. Describe the cause and effect of local winds (slope/valley and land/sea breeze) on wildland fire behavior.

5. Describe typical diurnal slope and valley wind patterns, and identify these temporal patterns on a topographic map.

6. Describe critical winds and their impact on wildland fire behavior.

7. Identify three ways topography can alter wind speed and direction.

Unit 7 Objectives

8. Describe general, local, 20-foot and mid flame winds, and their relationship to each other.

9. Adjust wind speeds based on topographic location and calculate mid-flame wind speeds for the three main fuel types.

Unit 7 Objectives

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