Fire acceleration is defined as the rate of increase in fire spread rate. It affects the amount of time required for a fire spread rate to achieve the theoretical steady state spread rate given 1) its existing spread rate, and 2) constant environmental conditions. This usage only applies to fire spread under constant environmental conditions (e.g. fuel moisture constant, winds constant, fuel availability constant). Other definitions of acceleration also exist (e.g. Cheney and Gould 1997) but these don't separate effects of changing fire environment from physical process of build-up rate. Changes in the fire environment that lead to increases in available fuel and spread rate are simulated explicitly as separate fire behavior processes by the simulation.
Fire acceleration rates are distinguished simply by "ignition type". Point source fires have slower acceleration than line source fires (Johansen 1987). These ignition types are used in FARSITE for convenience, although both types are really elements of a continuum of fire spread from a 1-dimensional source. Even this is simplistic, however, for most fires have 2-dimensional curving fronts with alternating convex and concave regions; small concave regions would be expected to have more rapid acceleration than convex regions as a result of different heat transfer ahead of the front. Similarly, fire acceleration from multiple separate fires with nearby fronts would also be increased. No 2-dimensional effects of fire shape on acceleration are addressed in FARSITE.
The lack of independence between portions of a fire front technically violates this assumption of Huygens' principle. Unlike light waves that interfere little among sources, fire propagation can interact along curving fronts and between multiple sources. It is not known what, if any, corrections are required to model fire growth and behavior for the practical applications intended for FARSITE.
Fire acceleration is currently implemented as a two step process using the model of the Canadian Forest Fire Behavior Prediction System (Alexander et al. 1992). The first calculation is always performed. It computes the final forward spread rate by accelerating the fire from its current rate toward the new equilibrium over the given time step. The average spread rate is computed from the distance traveled divided by the time step.
A second calculation becomes necessary only if the distance check was used to truncate the fire spread distance within the time step. Here, the average and final spread rates are calculated using the actual spread distance (the distance check) and initial rate of spread. This is done numerically using Newton's method to iterate travel time over the check distance.