Fire Behavior

Photosynthesis

Biomass is Destroyed

Four Phases of Combustion Process

1). Pre-ignition

Heat is required to raise a fuel to ignition temperature (600 degrees F)

Four Phases of Combustion Process

2). Ignition

Process where a rapid, exothermic reaction is initiated, which causes the material to undergo changes, producing temperatures greatly in excess of the ambient (Drysdale 1985).

Four Phases of Combustion Process

3). Combustion

Flaming=volatiles that are produced in the preheating phase ignite to form a visible flame

Smoldering=the remaining carbon may burn as a solid by surface oxidation

Four Phases of Combustion Process

4). Extinction

Opposite of ignition (termination of combustion)

Moisture content of fuel can play a large role

SMOKE

incomplete combustion where volatile products remain suspended as very small droplets of liquid plus residual carbonized particles that float in the air

Fire intensity-the rate at which a fire releases heat energy

Energy released/unit area/time (Btu/ft/sec)

BEHAVE FIRE MODEL

3 Basic Types of Fire Behavior

Fire Behavior Terminology

1). Running

2). Creeping

3). Smoldering

4). Spotting

Fire Behavior Terminology

5). Torching

6). Crowning

7). Flare-up

8). Blowup

Topography-- Slope Position

Topography-- Slope Position

Front Fire Backing Fire

Topography-- Slope Steepness

Pinyon-Juniper

Ponderosa Pine

Mixed Conifer

Spruce/Fir

Alpine

North South

5,000 ft6,000 ft7,000 ft

8,000 ft9,000 ft

10,000 ft

11,000 ft12,000 ft13,000 ft

14,000 ft

Topography-- Aspect and Elevation

Aspect

SOUTHSOUTH

NORTHNORTH

Heavy fuelsShadeMoist

Light fuelsSunnyDry

Topography-- Barriers

Topography-- Box Canyon

Chimney Effect- Occurs when unstable air conditions at the surface create a convection current through the canyon, drawing air in at the base of the canyon and exhausting it at the top

Radiant Heat Across Narrow Canyon

Spotting Across Narrow Canyon

1. insolation—solar radiation that is intercepted by Earth (surface and atmosphere)

Influenced by: a) angle of incoming radiation b) length of time exposure

Weather-Temperature

Weather-Temperature

patterns of heat gain/loss daily

Midnight 6 noon 6 Midnight

Coolest time

Hottest time-maximum amount absorbed

Temp Energy Flow

Spatial patterns on heat/cold

Day

Night

warm

cold

Weather-Temperature

The amount of humidity the air can hold at a specific temperature compared to the amount that it is actually holding is called the relative humidity.

Weather-Relative Humidity

Red line is fuel moisture--note lag time

WINDWARD LEEWARD

Rainshadow

Dewpoint

Chinook Winds

Adiabatic lapse rate-change in temperature with altitude due to pressure

Weather-Air Stability

Weather-Wind/Storm Relationships

Weather-Wind

horizontal movement of air

Weather-Types of Wind

Mountains Cause Channeling of Wind

Convection Column-rapidly rising superheated smoke can soar 35,000 feet or higher into the atmosphere and cause a rapid indraft of cooler air to the fire on the ground

Weather-Fire

El Nino is a disruption of the ocean-atmosphere system in the tropical Pacific that has implications for weather around the globe

1) Fuel-QuantityWays to Classify Fuels:

Oven dry weight of fuels in a given area

2) Fuel-Size and Shape

Duff is the organic forest horizons (fermentation and humus layers) that accumulate above the mineral soil

Litter is the surface layer consisting of freshly fallen leaves, needles, twigs, stems, and bark.

3) Fuel-Arrangement

Uniform Fuels

Patchy Fuels

Ladder Fuels

4) Fuel-Components

3) GROUND

2) SURFACE

1) AERIAL

How are fuels characterized?

1) Direct-sampling

2) Planar transect

3) Photo guides

4) Prediction models

Fuel-Moisture Content

Fuel moisture-percent of dry weight of fuel

Live fuel moisture- can range from 1-over 100 percentDead fuel moisture- often below 10 percent

Timelag Intervals-time required to lose 63 percent (2/3) of difference between original moisture content and equilibrium moisture content at a constant temperature and relative humidity

Fuel changes over time

1) Abrupt

2) Diurnal

3) Seasonal

4) Annual

• 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.

Indicators of Problem and Extreme Fire Behavior

Fuel IndicatorsFuel Indicators

Indicators of Problem and Extreme Fire Behavior

• Steep slopes.• Chutes, saddles, and box canyons which

provide conditions for “chimney effect.”• Narrow canyons may increase fire spread by

radiant heat and spotting.

Topography IndicatorsTopography 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.

Indicators of Problem and Extreme Fire Behavior

Weather IndicatorsWeather 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.

Indicators of Problem and Extreme Fire Behavior

Fire Behavior IndicatorsFire Behavior Indicators