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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."
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.
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.
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.