Vortex Phenomena

A vortex is a convective eddy that spins about either a horizontal or vertical axis. The air moves around a central axis as well as toward and along the axis. Vortices occur at a variety of scales in fires, varying from only a few inches or feet across that last a few seconds to several hundred feet in diameter and thousands of feet high. They play a significant role in movement of the fire front and in spreading fire beyond the fire’s perimeter, especially in large fires. For example, horizontal roll vortices are a major mechanism in crown-fire spread, and post-burn aerial photography indicates that unburned tree-crown streets are common with crown fire (Haines 1982).

Vortex formation often begins with horizontal rolls caused by wind speeds changing with height (vertical shear). When the horizontal vortex crosses a local region of buoyancy (as in a convectional updraft), a portion of the vortex is tilted up, and often split, creating two vertical eddies spinning in opposite directions. Vortex formation can also occur without outside forces such as wind shear if gradients of pressure or density occur within a plume in opposing directions. A vertical vortex intensifies as air moving into the vertical “pipe” is lifted up (or forced down), causing the vortex to stretch and narrow, further increasing the speed of air parcels within the column (Jenkins et al. 2001).

There appear to be two types of fire whirlwinds in the development of large vortices: one type occurs as a pair on the leeward side of the convection column and the other type is created after the entire convection column begins to rotate. The second type occurs in association with very intense fire that may be described as fire storms.