A fire whirl – also commonly known as a fire devil, or, (in many cases erroneously), as a fire tornado, firenado, fire swirl ,or fire twister – is a whirlwind induced by a fire and often made up of flame or ash. They usually start with a whirl of wind or smoke. 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 vortex that sucks in burning debris and combustible gases.
Fire whirls are sometimes colloquially called fire tornadoes, but are not usually classifiable as tornadoes as the vortex in most cases does not extend from the surface to cloud base. Also, even in such cases, even those fire whirls are not classic tornadoes, in that their vorticity derives from surface winds and heat-induced lifting, rather than a tornadic mesocyclone aloft.
Australia’s capital city, Canberra, on January 18th, 2003 experienced an actual tornado (like the ones found in the USA) that was produced by the strength of a wildfire. This was the first ever documented tornado that was produced by a wildfire. It was formed when the massive wildfire created a strong enough updraft to form a pyrocumulonimbus cloud. Pyrocumulonimbus clouds can have the same characteristics as cumulonimbus clouds, as well as producing similar weather, except they are formed from wildfires. The Australian wildfire provided the cloud with a strong enough updraft that it made the pyrocumulonimbus cloud classify as a supercell thunderstorm. The supercell became so strong that it spawned a tornado in the fire, causing more damage to be done.
A fire whirl consists of a burning core and a rotating pocket of air. A fire whirl can reach up to 2,000 °F (1,090 °C). Often, fire whirls are created when a wildfire or firestorm creates its own wind, which can turn into a vortex of fire. This causes the tall and skinny appearance of a fire whirl's core.
Most of the largest fire whirls are spawned from wildfires. They form when a warm updraft and convergence from the wildfire are present. They are usually 10–50 meters tall, a few meters wide, and last only a few minutes. However, some can be more than a kilometer tall, contain winds over 160 km/h, and persist for more than 20 minutes.
Fire whirls can uproot trees up to 15 metres (49 ft) tall. These can also aid the 'spotting' ability of wildfires to propagate and start new fires as they lift burning materials such as tree bark. These burning embers can be blown away from the fireground by the stronger winds aloft.
An extreme example of a fire tornado is the 1923 Great Kantō earthquake in Japan which ignited a large city-sized firestorm and produced a gigantic fire whirl that killed 38,000 people in fifteen minutes in the Hifukusho-Ato region of Tokyo.
Another example is the numerous large fire whirls (some tornadic) that developed after lightning struck an oil storage facility near San Luis Obispo, California on 7 April 1926, several of which produced significant structural damage well away from the fire, killing two. Many whirlwinds were produced by the four-day-long firestorm coincident with conditions that produced severe thunderstorms, in which the larger fire whirls carried debris 5 kilometers away.
A fire whirl, of reportedly uncommon size for New Zealand wildfires, formed on day three of the 2017 Port Hills fires in Christchurch. Pilots estimated the fire column to be 100 metres (330 ft) high.
There are currently three known types of fire whirls:
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