Understanding Fire Behavior Factors
There are variable factors that can contribute to sudden changes in fire behavior. The list below reviews the most common factors that indicate potential for sudden change.
Chimneys, chutes, gullies, and canyons: Any of these topographical depressions (no matter how slight) can draw the leading edge of the fire. Moreover, convection currents of heated gasses will be ahead of the fire in quantities that can kill.
Saddles: Saddles are at the top of canyons, therefore expect running fires to be drawn to saddles. Expect more fire intensity in the saddle than anywhere else along the ridge-top.
Mouths of canyons: During foehn wind driven fires, the fire behavior at the low end of canyons is similar to that in saddles during slope driven fires; i.e. you can expect the greatest intensity at those locations.
Mid-slope roads: If there is fuel below a mid-slope road, you can be overrun.
Aspect: Always note the aspect and the time of day to predict potential burning conditions for the rest of daylight hours.
Flashy fuels: If there are flashy fuels below, expect spot fires with a sudden ignition and a rapid rate of spread.
Low dead fuel moisture: Expect greater fire intensity.
Low live fuel moisture: Contributes to faster spread and greater intensity.
Shrub and timber fuels: These fuels below your position may create extreme fire intensity.
Winds: Expect sudden changes in slope and valley winds caused by topographic features or fire behavior; e.g. eddies, roll eddies and fire whirls.
Unstable air: Visible signs of unstable air may portend the possibility of large fire whirls and extreme fire behavior.
Temperature and moisture: Rising temperature, dropping RH, and dropping FFM may increase spot fires and rapid rates of spread.
Frontal systems and thunderstorms: As they form and/or approach, these systems may set the stage for sudden and extreme fire behavior changes.
Alignment of Forces
Any one of the above factors can lead to a sudden change in fire behavior that can catch you off guard. When your position includes several of these, the potential can be great. Two mutual aid companies were caught in an entrapment under these conditions in 1996. “All the forces, wind, slope, and preheated fuel were stacked and in alignment on the topography between the defensive position and the fire.”
Because some of these factors have no visible cues, and because some can cause a very rapid acceleration of fire spread, escape planning should always have an extra margin of safety to account for sudden changes and a rate of spread that is faster than predicted.