At present, spotting in FARSITE is simulated only from torching trees (Albini 1979). Maximum spot distances of embers from burning slash piles (Albini 1981), and line fires (Albini 1983a, 1983b, Morris 1987) are calculated in BEHAVE, but are not present in FARSITE. As originally implemented in BEHAVE, the spotting component of FARSITE is intended to compute the maximum spotting distance from a given point on a fire front if torching occurs. It is not intended to simulate the numbers of embers, exact locations embers would land, or locations of resulting spot fires.
Spotting is achieved as a 3 step process. The first step determines the maximum lofting height of cylindrical embers of 16 size classes (1/16" to 1 inch) that originate at the top of a tree or group of trees (see above). Some embers may be too heavy to be lofted by the given plume generated by the tree(s). The burning characteristics of a tree (flame height and duration) are determined by size and species of the trees (Albini 1979). For all lofted embers, the position, size, and time of lofting are stored until the end of a timestep.
After embers from all portions of all fire fronts are lofted in a given time step, each ember is iteratively descended until it is extinguished or contacts the ground. The iteration assumes that each ember falls at its terminal velocity determined by diameter and the time since it began falling. Using 50ft steps for vertical drop, the iteration accounts for change in the ground elevation, wind speed as a function of height above ground, and time of flight. Embers are assumed to follow the ambient wind direction. An ember will be extinguished if it is airborne longer than the burning duration determined by its size (Albini 1979). Above the canopy height, a logarithmic wind profile is assumed. This may overestimate spot distance if the terrain is not forest covered (Albini 1981). Below the canopy, the midflame wind speed is assumed vertically constant.
The third step involves ignition of spot fires and checking for impact of the embers within existing fire perimeters. At present, all embers burning at impact are considered ignitions. The probability of ignition (Bradshaw et al. 1984) is present, but not activated in this version of the simulation, given the intent to show locations of potential ignitions rather than their result.