Vortices are divided into two types, which are named based on the orientation of their axes of rotation to the ground. The vortices that rotate with their axes parallel to the ground are called horizontal vortices and those that rotate perpendicular to the ground are referred to as vertical vortices.

Horizontal Vortices

Horizontal vortices develop under extreme burning conditions. There are basically two types of horizontal vortices that develop relative to wildfires. The first occurs along the ground on the flanks of wildfires and can directly impact the safety of fireline personnel. The second type occurs in the convective column over the fire and can affect air operations near the fire.

These vortices tend to form more readily with low to moderate wind speeds over flat or gentle terrain.

Thermacam images of a horizontal vortices

Images were taken looking down on horizontal vortices within a fire.

This sequence of six images, taken 0.75 seconds apart, looking down on a horizontal vortices within a fire, comes from the University Corporation for Atmospheric Research (UCAR) Inframetrics Thermacam . The brightest yellow areas represent the hottest temperatures. FOD in each image refers to the "Finger Of Death". In frame 1, the arrow points to the starting point of the FOD in the upper right shoulder of the fire at the beginning time of 0.00 seconds. Frame 2 has arrow pointing to the FOD, which is beginning to form and burst forward at 0.75 seconds. The next image, frame 3, has the arrow showing that the FOD progresses further by 1.50 seconds. Frame 4 shows further elongation of the FOD at 2.25 seconds. By frame 5, the arrow shows that the FOD has extended to approximately 100 meters or 109 yards in just 3.00 seconds. Frame 6 shows the regression of the FOD to the main fire by 3.75 seconds.

This sequence shows that a "finger" of fire burst forward about 100 meters from the head of the fire at speeds exceeding 100 miles per hour, and then retreated back into the fire within three seconds. Seven horizontal rolls and capping vortices are also identified in the third frame. Horizontal rolls within fires have been hypothesized as occurring with crown fires. However, actual documentation has been limited.

A raging crown fire run

Use the video control above to play the movie.

Vertical Vortices

Commonly observed vertical vortices include dust devils and whirlwinds. When a vertical whirling mass of air includes the fire's flames, the vortex is called a fire whirl. We can further classify vertical vortices based on their triggering mechanisms. Thermally-driven vortices develop on hot days and in intense portions of the wildland fire. Wake-type vortices occur on the lee sides of physical obstructions to wind movement such as ridge tops and trees, and can also be seen on the edges of convective columns.

A fire whirl

Use the video control above to play the movie.