What Is a Fire Tornado?
A fire whirl – also commonly known as a fire devil, fire tornado, firenado, or fire twister – is a whirlwind induced by a fire and often made up of flame or ash. Fire whirls may occur when intense rising heat and turbulent wind conditions combine to form whirling eddies of air. These eddies can contract into a tornado-like structure that sucks in burning debris and combustible gases.
Fire whirls are sometimes colloquially called fire tornadoes, but are not literally tornadoes, in that their vorticity derives from surface winds and heat-induced lifting, rather than a tornadic mesocyclone aloft.
The literal fire tornado is a rare phenomenon, much more powerful and destructive than the ordinary fire whirl, in which the wildfire generates a pyrocumulonimbus cloud that then spawns a tornado. When this tornado forms in or enters the fire zone, it fans flames enormously, draws flame and burning debris into itself, and scatters burning debris over a wide area.
A fire tornado is generated by the fire’s own effect on local airflow, as with any fire whirl, but indirectly and by a different route. The fire also has to be enormous in order to create a pyrocumulonimbus cloud powerful enough for it to be classified as a supercell thunderstorm. Then the conditions have to be just right in order for a fire tornado to form.
A fire whirl consists of a core – the part that is actually on fire – and an invisible rotating pocket of air. A fire whirl can reach up to 2,000 °F (1,090 °C) – hot enough to potentially reignite ashes sucked up from the ground. Often, fire whirls are created when a wildfire or firestorm creates its own wind, which can turn into a spinning vortex of flame.
Combustible, carbon-rich gases released by burning vegetation on the ground are fuel for most fire whirls. 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. This causes the tall and skinny appearance of a fire whirl’s core.
Real-world fire whirls usually move fairly slowly. Fire whirls can set objects in their paths ablaze and can hurl burning debris out into their surroundings. The winds generated by a fire whirl can also be dangerous. Fire whirls can last for an hour or more, and they cannot be extinguished directly, they can uproot trees up to 49 ft (15m).
Fire tornadoes are usually 30-200ft (10-50m) and a few meters wide, and last only a few minutes. But some of the largest can be more than a half a mile tall, contain winds over 100mph, and persist for more than 20 minutes.
Most large fire tornadoes are formed when wildfire converges with a warm updraft of air, and wildfires can contain a number of fire whirls of varying intensity, size and duration. An extreme example is the 1923 Great Kantō earthquake in Japan which ignited a city-sized firestorm and produced an enormous fire whirl that killed 38,000 people in fifteen minutes.
A rash of large fire whirls developed after lightning struck an oil storage facility near San Luis Obispo, California on April 7, 1926, killing two. Thousands of whirlwinds were produced by the four-day-long firestorm – the larger of which carried debris 3 miles away.