Fire Behavior

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Fire Behavior
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Photosynthesis
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Biomass is Destroyed
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Four Phases of Combustion Process
1). Pre-ignition
Heat is required to raise a fuel to ignition
temperature (600 degrees F)
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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).
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Four Phases of Combustion Process
3). Combustion
Flamingvolatiles that are produced in the
preheating phase ignite to form a visible flame
Smolderingthe remaining carbon may burn as a
solid by surface oxidation
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Four Phases of Combustion Process
4). Extinction
Opposite of ignition (termination of
combustion) Moisture content of fuel can play a
large role
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SMOKE
incomplete combustion where volatile products
remain suspended as very small droplets of liquid
plus residual carbonized particles that float in
the air
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Fire intensity-the rate at which a fire releases
heat energy Energy released/unit area/time
(Btu/ft/sec)
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3 Basic Types of Fire Behavior
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Fire Behavior Terminology
1). Running 2). Creeping 3). Smoldering 4).
Spotting
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Fire Behavior Terminology
5). Torching 6). Crowning 7). Flare-up 8). Blowup
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Topography-- Slope Steepness
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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
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Radiant Heat Across Narrow Canyon
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Spotting Across Narrow Canyon
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Topography-- Aspect and Elevation
14,000 ft
13,000 ft
Alpine
12,000 ft
11,000 ft
Spruce/Fir
North
South
10,000 ft
Mixed Conifer
9,000 ft
8,000 ft
Ponderosa Pine
7,000 ft
6,000 ft
Pinyon-Juniper
5,000 ft
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Aspect
NORTH
Heavy fuels Shade Moist
SOUTH
Light fuels Sunny Dry
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Topography-- Slope Position
Backing Fire
Front Fire
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Topography-- Slope Position
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Topography-- Barriers
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Weather-Temperature
patterns of heat gain/loss daily
Temp
Energy Flow
Hottest time-maximum amount absorbed
Coolest time
Midnight 6 noon 6
Midnight
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Weather-Relative Humidity
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.
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Red line is fuel moisture--note lag time
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Weather-Wind/Storm Relationships
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Mountains Cause Channeling of Wind
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Weather-Fire
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
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El Nino is a disruption of the ocean-atmosphere
system in the tropical Pacific that has
implications for weather around the globe
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Ways to Classify Fuels
1) Fuel-Quantity
Oven dry weight of fuels in a given area
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2) Fuel-Size and Shape
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Duff is the organic forest horizons (fermentation
and humus layers) that accumulate above the
mineral soil
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Litter is the surface layer consisting of freshly
fallen leaves, needles, twigs, stems, and bark.
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3) Fuel-Arrangement
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Uniform Fuels
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Patchy Fuels
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Ladder Fuels
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Fuel-Moisture Content
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Fuel moisture-percent of dry weight of fuel
Live fuel moisture- can range from 1-over 100
percent Dead fuel moisture- often below 10 percent
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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
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Fuel changes over time
1) Abrupt 2) Diurnal 3) Seasonal 4) Annual
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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.

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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.

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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.

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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.