Fire Tornado

A fire whirl comes perilously close to homes during the Corona Fire on November 15, 2008, in Yorba Linda, California.

Fire tornadoes can set objects in their paths ablaze, and they can hurl burning debris out into their surroundings.

The winds generated by a fire tornado can also be dangerous. Large fire tornadoes can create wind speeds of more than a hundred miles (160 kilometers) an hour—strong enough to knock down trees.

A twisting column of flame arcs toward the sky during a 2006 wildfire in Los Padres National Forest near Castic, California

Fire tornadoes can last for an hour or more, and they can't be extinguished directly, Forthofer said.

Visitors watch an artificial fire tornado created by several air-jet ventilators at the Phaeno Science Center in Wolfsburg, Germany, in August 2007.

Real-world fire whirls aren't so stationary, but they won't win any speed records. "They usually move fairly slow, about as fast as you can walk or even slower," RMRC's Forthofer 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."

A wildfire-induced tornado of hot ash dances across a ridgetop near Rancho Santa Margarita, California, in May 2002.

 The temperature inside the core of a fire tornado can reach up to 2,000 degrees Fahrenheit (1,093 degrees Celsius)—hot enough to potentially reignite ashes sucked up from the ground, Forthofer said
"We're not totally sure about that, but it's one theory," he said. "It's like if you've ever seen anyone try to burn cooking flour: If you puff it up enough in the air, you can actually burn it. But as it sits compacted in a lump, it won't burn."

A fire tornado swirls near a chimney on the roof of a burning house in an unidentified location

Fire tornadoes occur when intense heat and turbulent wind conditions combine to form whirling eddies of air. These eddies can tighten into a tornado-like structure that sucks in burning debris and combustible gases, RMRC's Forthofer explained. A fire tornado consists of a core—the part that is actually on fire—and an invisible pocket of rotating air that feeds fresh oxygen to the core.
The core of a typical fire tornado is usually about 1 to 3 feet (0.3 to 0.9 meter) wide and 50 to 100 feet (15 to 30 meters) tall. But under the right conditions, very large fire tornadoes—several tens of feet wide and more than a thousand feet (300 meters) tall—can form, Forthofer said. 

"These really large-scale fire tornadoes occur at least once every year somewhere in the U.S.," he added.  

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.