Introduction to Wildland Fire Behavior S-190 Unit II

Unit 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 02-01(1 of 2)-S190

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 02-01(2 of 2)-S190

Differential Heating

Group 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 S190

General or Gradient Winds S190 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 Front

Cold 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 S190

Cold Front

Cold Front Winds S190

Indicators 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 S190

Cold Fronts are Potentially Dangerous 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 S190

Fire Before Cold Front Passage S190

Fire After Cold Front Passage S190

Foehn 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 S190

Subsidence S190

Examples of Foehn Winds S190

Thunderstorm S190 A storm produced by a cumulonimbus cloud Always accompanied by: –Lightning –Thunder –Strong gusty winds

Visual Indicators of Thunderstorm S190 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

Thunderstorm S190

Why 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 S190

Sea Breeze Land Breeze Sea Breeze Land Breeze S190

Slope Winds S190 NightDay

Valley Winds S190 NightDay

Temperature/Relative Humidity Chart S190 NoonMidnightNoon Maximum Minimum Relative Humidity Temperature

Visual Indicators of Unstable Air S190 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 Clouds

Dust Devil S190

Obvious Unstable Air

Visual Indicators of Stable Air S190 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 Clouds

Inversion S190

Thermal Belt S190 40° 45° 50° 45° 40°

Indicators of Problem and Extreme Fire Behavior: Fuel 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 snags S190

Fire 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. 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. 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

Indicators of Problem and Extreme Fire Behavior: Topography 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 S190

Steep 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 Valleys Remember, narrow canyons + steep slopes = death traps!

Indicators of Problem and Extreme Fire Behavior: Weather 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 column S190

Aftermath of a wind-driven fire

Firewhirls Generated by intense fires Can pick up large burning embers and toss them far across the fireline causing spot fires S190

Indicators of Problem and Extreme Fire Behavior: Fire Behavior Indicators Keep an eye on the smoke column –Indicates direction of fire spread, location of 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 trees or shrubs Frequent spot fires occurring Firewhirls beginning to develop inside the main fire Crown fires S190

Unit 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 (1 of 2)-S190

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 (2 of 2)-S190