Firewhirls

The heat generated by fires produces extreme instability in the lower air and may cause violent firewhirls. Such firewhirls have been known to twist off trees more than 3 feet in diameter. They can pick up large burning embers, carry them aloft, and then spew them out far across the fireline and cause numerous spot fires. At times, the firewhirls move bodily out of the main fire area, but as soon as they do the flame dies out and they become ordinary whirlwinds moving across the landscape.

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. A favored area for firewhirls is the lee side of a ridge where the heated air from the fire is sheltered from the general winds. Mechanical eddies produced as the wind blows across the ridge can serve as the triggering mechanism to initiate the whirl. The wind may add to the instability by bringing in cool air at higher levels over the fire-heated air on the lee side. Air streams of unequal speeds or from different directions in adjacent areas can mechanically set off firewhirls in fire-heated air. Firewhirls have also been observed in relatively flat terrain. In these cases the whirls seem to start when a critical level of energy output has been reached by a portion of the fire.