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.
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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.
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"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.