FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

significantly with differences in moisture content.
d. Other predictable aspects of fire behavior.
(1) Probability of ignition. The probability of ignition, expressed as a
percentage, is an estimate of the probability that a spark or firebrand landing on
representative fuels will start a fire (Rothermel 1983). It is based on the amount of
heat required to bring fine fuel to ignition temperature. Model inputs are fine fuel
moisture, ambient air temperature, and the amount of shade.
(2) Maximum spotting distance. For many fuels situations, it is possible to make
reasonably accurate estimates of the maximum distance to which fire may spot
ahead by airborne embers (Rothermel 1983). The inputs required include the
source of the embers, i.e. whether it is burning piles or trees; the species of tree,
and their size and shape; the topography at and downwind from the fire; and the
20 foot windspeed. The model calculates the farthest distance a live ember is
likely to be carried. It does not estimate how many burning embers will be lofted,
or if the ember will ignite a spot fire. However, a combination of maximum spotting
distance with the probability of ignition provides a workable idea of how far a fire
may spot and the probability that it will cause a new fire.
(3) Crown fires.
(a) Classes. Van Wagner (1977) grouped crown fires into three classes based
upon their dependence on the behavior of the surface fire.
i. Passive crown fires are those in which trees torch as individuals, ignited by the
surface fire. These fires spread at essentially the same rate as surface fires.
Trees torch within a few seconds with the entire crown enveloped in flames from
its base to the top.