Doyle Rice / USA Today
Some of the ferocious wildfires this week in southern California have spawned fire whirls, which have also been dubbed firenadoes or fire tornadoes.
A fire whirl is a "spinning vortex column of ascending hot air and gases rising from a fire and carrying aloft smoke, debris, and flame," according to the Bureau of Land Management's Glossary of Wildland Fire Terms. "Fire whirls range in size from less than one foot to more than 500 feet in diameter. Large fire whirls have the intensity of a small tornado."
These fire whirls form in any size fire, but they are most destructive in large fires. Created by cool air rushing to take the place of hot air, the whirl's spinning can hurl embers and sparks great distances. Whirls usually intensify a wildfire.
"There are little vortices all over the place on the edge of where the fire is forming, just like swirls on the gust front of a thunderstorm," according to meteorologist Harold Brooks of the National Severe Storms Laboratory.
"Put vertical motion on top of them (there's lots of that available around a fire) and it will rapidly concentrate and intensify the rotation," he said.
Fire whirls are also more likely to occur where winds are forced to change directions, such as near a grove of trees.