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)
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 Steepness
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
Topography-- Aspect and Elevation
14,000 ft
13,000 ft
12,000 ft
11,000 ft
10,000 ft
9,000 ft
8,000 ft
7,000 ft
6,000 ft
5,000 ft
Alpine
North
Spruce/Fir
Mixed Conifer
Ponderosa Pine
Pinyon-Juniper
South
Aspect
NORTH
Heavy
fuels
Shade
Moist
SOUTH
Light fuels
Sunny
Dry
Topography-- Slope Position
Front Fire
Backing Fire
Topography-- Slope Position
Topography-- Barriers
Weather-Temperature
patterns of heat gain/loss daily
Temp
Energy Flow
Hottest time-maximum
amount absorbed
Coolest time
Midnight
6
noon
6
Midnight
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.
Red line is fuel moisture--note lag time
Weather-Wind/Storm Relationships
Mountains Cause Channeling of
Wind
Weather-Fire
Convection Columnrapidly 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
El Nino is a
disruption of
the oceanatmosphere
system in
the tropical
Pacific that
has
implications
for weather
around the
globe
Ways to Classify Fuels:
1) Fuel-Quantity
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
Fuel-Moisture Content
Fuel moisturepercent of dry
weight of fuel
Live fuel moisture- can range from
1-over 100 percent
Dead 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
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.
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.
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.
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.