S290 Unit 7
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7-1-S290-EPUnit 7 Wind Systems
Unit 7Wind Systems
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7-2-S290-EPUnit 7 Wind 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.
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7-3-S290-EPUnit 7 Wind Systems
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
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7-4-S290-EPUnit 7 Wind Systems
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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7-5-S290-EPUnit 7 Wind Systems
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
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7-6-S290-EPUnit 7 Wind Systems
Winds That Influence Wildland Fire Behavior
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7-7-S290-EPUnit 7 Wind Systems
Wind Direction
• The direction from which the wind is blowing
• North wind blows from north to south
Don’t forget!!!
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7-8-S290-EPUnit 7 Wind Systems
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.
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7-9-S290-EPUnit 7 Wind Systems
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.
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7-10-S290-EPUnit 7 Wind Systems
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.
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7-11-S290-EPUnit 7 Wind Systems
• How the earth’s general wind circulation would appear if the earth did not rotate on its axis.
General Winds
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7-12-S290-EPUnit 7 Wind Systems
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.
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7-13-S290-EPUnit 7 Wind Systems
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.
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7-14-S290-EPUnit 7 Wind Systems
General Wind Circulation
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7-15-S290-EPUnit 7 Wind Systems
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
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7-16-S290-EPUnit 7 Wind Systems
General WindJet Stream
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7-17-S290-EPUnit 7 Wind Systems
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
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7-18-S290-EPUnit 7 Wind Systems
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
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7-19-S290-EPUnit 7 Wind Systems
High Pressure– Pressure
increases towards the center of the high
– The highest pressure is located at the center of the cell
General Wind Highs and Lows
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7-20-S290-EPUnit 7 Wind Systems
General Wind Highs and Lows
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7-21-S290-EPUnit 7 Wind Systems
7-21-S290-EP
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7-22-S290-EPUnit 7 Wind Systems
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7-23-S290-EPUnit 7 Wind Systems
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7-24-S290-EPUnit 7 Wind Systems
L
H H
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7-25-S290-EPUnit 7 Wind Systems
General WindHighs and Lows vs. Ridges and Valleys
• High pressure ridge– Similar to ridges on a topo
map
• Low pressure trough– Similar to valleys on a topo
map
• Contours are similar to isobars– Contour gradient
proportional to slope steepness
– Isobar gradient proportional to wind speed
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7-26-S290-EPUnit 7 Wind Systems
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7-27-S290-EPUnit 7 Wind Systems
Exercise 1
Jet StreamsHigh and Lows
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7-28-S290-EPUnit 7 Wind Systems
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
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7-29-S290-EPUnit 7 Wind Systems
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
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7-30-S290-EPUnit 7 Wind Systems
Local WindsUpslope/Upvalley
Early to Mid-Morning- 3 to 8 mph
Late Morning andAfternoon- 10 to 15 mph
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7-31-S290-EPUnit 7 Wind Systems 7-31-S290-EP
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7-32-S290-EPUnit 7 Wind Systems
Local WindsUpslope/Upvalley Transition
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7-33-S290-EPUnit 7 Wind Systems
Local WindsDownslope/Downvalley
Late Evening and Overnight- 5 to 10 mph
Late Afternoon and
Evening- 2 to 5 mph
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7-34-S290-EPUnit 7 Wind Systems
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.
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7-35-S290-EPUnit 7 Wind Systems
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
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7-36-S290-EPUnit 7 Wind Systems
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
HH
AtlanticOcean
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7-37-S290-EPUnit 7 Wind Systems
Local WindsSea Breeze
1998 Perry Fire
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7-38-S290-EPUnit 7 Wind Systems
Slope & Valley WindExercise 2
A
B
C
D
E
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7-39-S290-EPUnit 7 Wind Systems
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
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7-40-S290-EPUnit 7 Wind Systems
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
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7-41-S290-EPUnit 7 Wind Systems
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
Critical WindsCold Front Winds
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7-42-S290-EPUnit 7 Wind Systems
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
Critical WindsCold Front Winds
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7-43-S290-EPUnit 7 Wind Systems
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
Critical WindsCold Front Winds
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7-44-S290-EPUnit 7 Wind Systems
Critical WindsCold Front Winds
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7-45-S290-EPUnit 7 Wind Systems
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
Critical WindsCold Front Winds
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7-46-S290-EPUnit 7 Wind Systems
Exercise 3
Critical WindsCold Front Winds
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7-47-S290-EPUnit 7 Wind Systems
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
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7-48-S290-EPUnit 7 Wind Systems
Critical WindsFoehn Winds
Foehn Winds• Chinook• Santa Ana • Mono• Wasatch• East• North
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7-49-S290-EPUnit 7 Wind Systems
Chinook Winds• Steep pressure
gradient forms between high pressure on the windward slope and low pressure on the lee slope
Critical WindsFoehn Winds
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7-50-S290-EPUnit 7 Wind Systems
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.
Critical WindsFoehn Winds
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7-51-S290-EPUnit 7 Wind Systems
Santa Ana Wind Storm• Romero fire October 10,
1971
• Note the sudden temperature rise and RH drop when the Santa Ana winds develop
Critical WindsSanta Ana Wind Storm
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7-52-S290-EPUnit 7 Wind Systems
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
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7-53-S290-EPUnit 7 Wind Systems
Two characteristicsimportant to fire
weather
• Lightning
• Indraft and downdraft winds are most important
Critical WindsThunderstorm Winds
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7-54-S290-EPUnit 7 Wind Systems
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
Critical WindsThunderstorm Winds
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7-55-S290-EPUnit 7 Wind Systems
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
Critical WindsThunderstorm Winds
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7-56-S290-EPUnit 7 Wind Systems
Outflow Winds• Outflow wind strongest
in the direction the storm is moving
• Outflow wind weakest in the opposite direction the storm is moving
Critical WindsThunderstorm Winds
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7-57-S290-EPUnit 7 Wind Systems
Outflow Winds• Typically spread radially
5 to 10 miles
• Topography can alter the direction of wind and speed
• Channeled through valleys, canyons, and drainages
Critical WindsThunderstorm Winds
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7-58-S290-EPUnit 7 Wind Systems 7-58-S290-EP
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7-59-S290-EPUnit 7 Wind Systems
Outflow Winds• Spread more
evenly over a flat plain
• Note the gust front located over the Texas Panhandle
Critical WindsThunderstorm Winds
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7-60-S290-EPUnit 7 Wind Systems
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
Critical WindsThunderstorm Winds
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7-61-S290-EPUnit 7 Wind Systems
Downdraft Indicators• Virga• Rain shaft • Dust cloud
Critical WindsThunderstorm Winds
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7-62-S290-EPUnit 7 Wind Systems
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
Critical WindsThunderstorm Winds
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7-63-S290-EPUnit 7 Wind Systems
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
Critical WindsThunderstorm Winds
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7-64-S290-EPUnit 7 Wind Systems
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7-65-S290-EPUnit 7 Wind Systems
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
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7-66-S290-EPUnit 7 Wind Systems
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
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7-67-S290-EPUnit 7 Wind Systems
• 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
CA
Critical WindsLow-Level Jets
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7-68-S290-EPUnit 7 Wind Systems
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
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7-69-S290-EPUnit 7 Wind Systems
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
Critical WindsWhirlwinds
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7-70-S290-EPUnit 7 Wind Systems
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
Critical WindsWhirlwinds
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7-71-S290-EPUnit 7 Wind Systems
THREE WAYS TOPOGRAPHY CAN ALTER THE SPEED AND DIRECTION
OF THE WIND
• Mechanical • Turbulent • Frictional
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7-72-S290-EPUnit 7 Wind Systems
Ways Topography Alters WindMechanical or Diverting Effects
Directional Channeling• Wind is channeled through valleys and drainages
• Airflow follows prominent terrain features
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7-73-S290-EPUnit 7 Wind Systems
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
Ways Topography Alters WindMechanical or Diverting Effects
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7-74-S290-EPUnit 7 Wind Systems
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 WindMechanical or Diverting Effects
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7-75-S290-EPUnit 7 Wind Systems
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
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7-76-S290-EPUnit 7 Wind Systems
Strong Winds in Canyons• Eddies can form at the
confluence of tributaries
• Spur ridges can be turbulent
Ways Topography Alters WindTurbulent Effects
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7-77-S290-EPUnit 7 Wind Systems
Thermal Turbulence• Caused by differential
heating from varying land surfaces
• Can disrupt low-level wind flow
Ways Topography Alters WindTurbulent Effects
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7-78-S290-EPUnit 7 Wind Systems
Obstructions or Irregularities• Low-level wind flow can be
disrupted by ground level obstructions or irregularities– Trees– Cliffs– Valleys
Ways Topography Alters WindTurbulent Effects
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7-79-S290-EPUnit 7 Wind Systems
Frictional Drag• Slows winds• Strength
dependent on varying surface roughness
Ways Topography Alters WindFrictional Drag
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7-80-S290-EPUnit 7 Wind Systems
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
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7-81-S290-EPUnit 7 Wind Systems
20–Foot Surface Wind
20-FootSfc Wind=
General WindComponent + Local Wind
Component
Wind AdjustmentsCalculating 20-Foot Surface Winds
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7-82-S290-EPUnit 7 Wind Systems
Wind AdjustmentsCalculating 20-Foot Surface Winds
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7-83-S290-EPUnit 7 Wind Systems
Wind AdjustmentsCalculating 20-Foot Surface Winds
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7-84-S290-EPUnit 7 Wind Systems
Wind AdjustmentsCalculating 20-Foot Surface Winds
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7-85-S290-EPUnit 7 Wind Systems
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
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7-86-S290-EPUnit 7 Wind Systems
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?
Wind AdjustmentsFor Topographic Locations
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7-87-S290-EPUnit 7 Wind Systems
• 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.
1/2 X
1 X8 mph
4 mph
8 mph
Wind AdjustmentsFor Topographic Locations
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7-88-S290-EPUnit 7 Wind Systems
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
1/2 X
1X 3/4X
1/4X
20 mph
20 mph 15 mph5 mph10 mph
Wind AdjustmentsFor Topographic Locations
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7-89-S290-EPUnit 7 Wind Systems
Wind Flowing over Hills andCanyons or Valleys
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7-90-S290-EPUnit 7 Wind Systems
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
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7-91-S290-EPUnit 7 Wind Systems
Exercise 4
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7-92-S290-EPUnit 7 Wind 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.
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7-93-S290-EPUnit 7 Wind Systems
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
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7-94-S290-EPUnit 7 Wind Systems
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