FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

c. Air mass. Weather components such as temperature, relative humidity,
windspeed and direction, cloud cover, precipitation amount and duration, and
atmospheric stability are all elements of the air mass. These values can change
quickly over time, and significantly with differences in aspect and elevation. The
air mass affects fire both by regulating the moisture content of fuel (discussed in
Chapter III.B.4.), and by its direct effect on the rate of combustion. The following
is a brief discussion of the effect of air mass factors on fire behavior and
characteristics.
(1) Temperature. Atmospheric temperature affects fuel temperature. The ease of
ignition, the amount of heating required to raise fuel to ignition temperature (320
C.; 608 F.) (Burgan and Rothermel 1984), depends on initial fuel temperature.
The most important effect of temperature, however, is its effect on relative
humidity and hence on dead fuel moisture content. (See Chapter III.B.4.).
(2) Windspeed. Wind has a significant effect on fire spread. It provides oxygen to
the fuel and, combined with slope, determines which way the fire moves. Wind
tips the flame forward and causes direct flame contact with fuel ahead of the fire
(Burgan and Rothermel 1984). These fuels are preheated and dried by this
increased transfer of radiant and convective heat. Windspeed has the most
influence on fire behavior in fuel types with a lot of fine fuels, such as grasslands.
2. Combustion Process.
a. Two stage process. Within a wildland fire, the processes of pyrolysis and
combustion occur simultaneously (Ryan and McMahon 1976 in Sandberg et al.
1978).
(1) Pyrolysis. When first heated, fuels produce water vapor and mostly
noncombustible gases (Countryman 1976). Further heating initiates pyrolysis, the
process by which heat causes chemical decomposition of fuel materials, yielding
organic vapors and charcoal (ibid.). At about 400F. (204 C)., significant amounts
of combustible gases are generated. Also at this temperature, chemical reactions
start to produce heat, causing pyrolysis to be self-sustaining if heat loss from the
fuel is small. Peak production of combustible products occurs at when the fuels
are about 600 F. (316 C.) (ibid.).
(2) Combustion. Combustion is the process during which combustible gases and
charcoal combine with oxygen and release energy that was stored in the fuel
(Countryman 1976) as heat and light.
b. Phases of combustion. The following summary is derived from Ryan and
McMahon (1976 in Sandberg et al. 1978), except where noted. For a more
complete discussion of the phases of combustion, see Sandberg et al. (1978).
(1) Pre-ignition phase. In this phase, heat from an ignition source or the flaming