Firefighters watch a "fire tornado" wreathed with dust and smoke as it swirls on the south slope of Hawaii's Mauna Kea volcano Sunday. The fiery column was spawned during a 1,400-acre (566-hectare) brush fire triggered by regional drought.
Also known as fire whirls, fire devils, or even firenados, these whirlwinds of flame are not really rare, just rarely documented, said Jason Forthofer, a mechanical engineer at the U.S. Forest Services's Missoula Fire Sciences Laboratory in Montana.
For instance, fire tornadoes were recently reported in the state of São Paulo, Brazil, where a three-month long drought has also led to several brush fires.
Forthofer studies fire tornadoes with the aim of protecting firefighters.
"If we can identify conditions that are conducive to fire whirls, that would be a heads-up for firefighters, because there have been some [people] that have been burned by them," he said.
A fire tornado rises from burning peat on a farm in Bangor, U.K., in 2008.
Combustible, carbon-rich gases released by burning vegetation on the ground are fuel for most fire tornadoes, Forthofer said. "The vegetation on the ground heats up enough to release gas, but some of the gas can't combust, because it doesn't have enough oxygen around it." When sucked up by a whirl of air, this unburned gas travels up the core until it reaches a region where there is enough fresh, heated oxygen to set it ablaze. That's why the flames in a fire tornado's core look so tall and skinny, Forthofer said.
"The [gases] can't burn until they mix with enough oxygen, and that might not happen until way up above the ground."
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