A look at the Firenado phenomenon.
Whipping winds and blazing brush led to a rare occurrence in San Diego Wednesday, as residents and firefighters witnessed a so-called firenado ripping across a flaming hillside.
Triggered when ground-level winds contact fire, the cyclones of fire shoot up to 100 feet as they suck in debris and spit out burning embers.
Researchers have found evidence of firenados - also called fire whirls - going back more than a century, including during the Great Chicago Fire.
They have been responsible for severe injuries and even deaths, and are often quick-moving and burn out in just a few minutes.
The exact conditions that cause firenados have been difficult to pin down.
After a lengthy study, researchers from the US Department of Agriculture concluded varying factors could create conditions for a firenado, and they are difficult to predict. The study's authors noted they were driven to examine the fire whirls because of the extreme danger they pose to firefighters.
"These factors interact in complex ways, and it is doubtful that firefighters will ever have very accurate predictive tools to foresee whirl formation, especially in a timely manner to make real-time decisions," said the study, published in the March 2011 Journal of Combustion.