Fire Whirls

Fire whirls are a rare but potentially catastrophic form of fire. These buoyant fire plumes are columns of ascending air with rotational motion which carry aloft smoke, debris, and flames greatly increasing the rate of fire spread (Pyne et al. 1996). Fire whirls range from a foot or two in diameter to small tornadoes in size and intensity (Brown and Davis 1973). Once a fire whirl leaves the influence of the main fire it quickly loses intensity and becomes an ordinary whirlwind (Schroeder and Buck 1970).

In order for a fire whirl to exist, there must be an organized source of angular momentum, either from wind shear or from the fire’s convection column, which in turn creates large swirl velocities as air is entrained into the fire plume (Battaglia et al. 2000). Firewhirls occur most frequently where heavy concentrations of fuels are burning and a large amount of heat is being generated in a small area. Mechanical forces are often present which serve as triggering mechanisms to start the whirl (Schroeder and Buck 1970). A common area for firewhirls is the lee side of a ridge where the heated air from the fire is sheltered from general winds. Mechanical eddies produced as the wind blows across the ridge can serve as the triggering mechanism to initiate the firewhirl. On flat terrain they tend to occur on the lee side of the fire near the outside edges of the front (see Vortex Phenomena).

Even in small fires that arent exhibiting extreme fire behavior, fire whirls can be created by lighting patterns. A rotational motion can be generated when two lines of fire come together, particularly if they approach at right angles. If large firewhirls are created, fire control problems can result.