IntroductioIntroduction to n to

Wildland Wildland Fire Fire

BehaviorBehavior

S-190S-190

Unit IIUnit II

Unit 2 ObjectivesUnit 2 Objectives

• List the indicators of an approaching cold front and describe what wind changes to expect

• List three common foehn wind conditions and the areas in which they occur

• Identify a thunderstorm and describe how and when it is dangerous

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Unit 2 Objectives (cont.)Unit 2 Objectives (cont.)

• Describe the daily cycle of slope and valley winds

• Describe the effect relative humidity has on wildland fire behavior

• Identify the wildland fire environment indicators that can produce problem and extreme fire behavior

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Differential HeatingDifferential Heating

Group 2 — WeatherGroup 2 — Weather

• Temperature

• Wind– Increases supply of oxygen– Drives convective heat into adjacent fuels– Influences direction of spread and spotting– Carries away moist air replacing it with drier air– Dries fuels– Raises fuel moisture if the air contains moisture

• Relative humidity– As RH increases, fuel moisture increases

• Precipitation– Increases fuel moisture

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General or Gradient WindsGeneral or Gradient Winds

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• Large scale winds caused by high and low pressure systems, but generally influenced and modified in the lower atmosphere by terrain

• Everyone is most familiar with them from the nightly news weather reports

• In short & simple terms, classed into warm fronts and cold fronts

Warm FrontWarm Front

Cold FrontCold Front

• A cold front is the boundary line between a cooler air mass which is replacing a warmer air mass

• Cold air is more dense and will hug the ground more than a warm front

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Cold FrontCold Front

Cold Front WindsCold Front Winds

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Indicators of a Cold Front PassageIndicators of a Cold Front Passage

• Line of cumulus clouds approaching from the west or northwest

• Large clouds of dust can precede the front

• Shift in winds from the southeast, south, to the southwest, and increase in velocity

• Winds will be strongest, erratic, and gusty as the front reaches you

• Winds will continue to shift as the front passes, generally resulting in strong, gusty, cool winds out of the west and northwest

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Cold Fronts are Potentially Cold Fronts are Potentially DangerousDangerous• Wind direction will abruptly change

• Strong southerly winds ahead of front will drive the fire head to the north or northeast

• Winds shifting to west or northwest after front passes will drive fire head to the east or southeast

• Rapid drop in relative humidity within 24 hours of front passage

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Fire Before Cold Front PassageFire Before Cold Front Passage

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Fire After Cold Front PassageFire After Cold Front Passage

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Foehn WindFoehn Wind

• Also called gravity winds, dry wind with strong downward components, characteristic of mountainous regions

• As the wind moves over long distances, air friction warms the moving air up and loses even more moisture

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SubsidenceSubsidence

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Examples of Foehn WindsExamples of Foehn Winds

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ThunderstormThunderstorm

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• A storm produced by a cumulonimbus cloud

• Always accompanied by:– Lightning– Thunder– Strong gusty winds

Visual Indicators of ThunderstormVisual Indicators of Thunderstorm

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• Tall, building cumulus cloud

• Cauliflower appearance of cloud

• Dark, flat base

• Virga or rain falling from bottom of cloud

• Ice crystal top usually in anvil shape with fuzzy appearance

ThunderstormThunderstorm

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Why a Thunderstorm is DangerousWhy a Thunderstorm is Dangerous

• Produces strong, gusty surface winds affecting direction of fire spread

• Downward winds will be erratic, but always away from the thunderstorm

• Thunderstorms also produce lightning, which can be dangerous

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Sea BreezeSea Breeze Land Land BreezeBreeze

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Slope WindsSlope Winds

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NightDay

Valley WindsValley Winds

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NightDay

Temperature/Relative Humidity Temperature/Relative Humidity ChartChart

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Noon Midnight Noon

Maximum

Minimum

Relative Humidity

Temperature

Visual Indicators of Unstable AirVisual Indicators of Unstable Air

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Relatively Cold

Relatively Warm

• Clouds grow vertically and smoke rises to great heights

• Cumulus-type clouds

• Gusty winds

• Good visibility

• Dust devils and firewhirls

Cumulus CloudsCumulus Clouds

Dust DevilDust Devil

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Obvious Unstable AirObvious Unstable Air

Visual Indicators of Stable AirVisual Indicators of Stable Air

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Relatively Cold

Relatively Warm

• Clouds in layers, no vertical motion

• Stratus-type clouds

• Smoke column drifts apart after limited rise

• Poor visibility in lower levels due to accumulation of haze and smoke

• Fog layers

• Steady winds

Stable Air CloudsStable Air Clouds

InversionInversion

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Thermal BeltThermal Belt

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40°

45°

50°

45°

40°

IndicatorsIndicators of Problem and Extreme of Problem and Extreme Fire Behavior: Fire Behavior: Fuel IndicatorsFuel Indicators• Unusually dry fuels

• Large amount of light fuel– shrubs, grass, needles

• Fuels exposed to direct sunlight

• Fuels dried by prolonged drought

• Ladder fuels that allow a surface fire to move into the crowns of shrubs or trees

• Crown foliage dried by surface fire

• Concentration of snags02-26-S190

Fire in Timber AreasFire in Timber Areas

• Fire normally stays on the surface unless there is enough heat or ladder fuels to move into the aerial fuels

• When the fire moves into the crowns, the situation changes dramatically

Torching vs. CrowningTorching vs. Crowning

• The term “Torching” refers to a single tree or a group of trees that rapidly burn off

• Torching stays in one spot; it’s isolated aerial fuels that cant jump to other aerial fuels

Torching vs. CrowningTorching vs. Crowning

• “Crowning” or a “Crown Fire” is where the fire is able to take all of the aerial fuels in the area, and keep going

• Moves very fast, extreme heat, and will continue until fuels, weather or topography make a change

IndicatorsIndicators of Problem and Extreme of Problem and Extreme Fire Behavior: Fire Behavior: Topography indicatorsTopography indicators• Steep slopes

• Chutes, saddles, and box canyons which provide conditions for “chimney effect”

• Narrow canyons may increase fire spread by radiant heat and spotting

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Steep SlopesSteep Slopes

• Slopes with the wind direction going uphill with the direction of the slope is a recipe for a fast moving hot fire

Canyons and ValleysCanyons and Valleys

• Remember, narrow canyons + steep slopes = death traps!

IndicatorsIndicators of Problem and Extreme of Problem and Extreme Fire Behavior: Fire Behavior: Weather IndicatorsWeather Indicators• Strong Wind

• Sudden changes in wind direction and velocity due to weather fronts

• High, fast-moving clouds may indicate unusual surface winds

• Unexpected calm may indicate wind shift

• Thunderstorms above or close to the fire

• Unusually high temperatures and low relative humidity

• Dust devils and whirlwinds developing

• Bent smoke column02-28-S190

Aftermath of a wind-driven fireAftermath of a wind-driven fire

FirewhirlsFirewhirls

• Generated by intense fires

• Can pick up large burning embers and toss them far across the fireline causing spot fires

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IndicatorsIndicators of Problem and Extreme of Problem and Extreme Fire Behavior: Fire Behavior: Fire Behavior Fire Behavior IndicatorsIndicators• Keep an eye on the smoke column

– Indicates direction of fire spread, locationof spot fires, and changes in fire intensity

• Many simultaneous fires starting or smoldering fires beginning to pick-up in intensity

• Fire begins to torch small groups of treesor shrubs

• Frequent spot fires occurring

• Firewhirls beginning to develop insidethe main fire

• Crown fires02-29-S190

Unit 2 ObjectivesUnit 2 Objectives

• List the indicators of an approaching cold front and describe what wind changes to expect

• List three common foehn wind conditions and the areas in which they occur

• Identify a thunderstorm and describe how and when it is dangerous

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Unit 2 Objectives (cont.)Unit 2 Objectives (cont.)

• Describe the daily cycle of slope and valley winds

• Describe the effect relative humidity has on wildland fire behavior

• Identify the wildland fire environment indicators that can produce problem and extreme fire behavior

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