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Fire whirl

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Article Information
Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
WX
Tag(s) Weather Phenomena

Description

A fire whirl, also commonly known as a fire devil, or, as a fire tornado, firenado, fire swirl, or fire twister, is a whirlwind induced by a fire and often (at least partially) composed of flame or ash. These start with a whirl of wind, often made visible by smoke, and may occur when intense rising heat and turbulent wind conditions combine to form whirling eddies of air. These eddies can contract a tornado-like vortex that sucks in debris and combustible gases.

Fire whirl - Source: Wikicommons

Even though a fire whirl is sometimes informally known as a firenado, it is not frequently classified as a tornado since its vortex does not always stretch from the base of the cloud to the ground. Fire whirls are not classic tornadoes since their vorticity derives from the temperature-induced lifting and surface winds instead of the tornadic mesocyclone aloft. A mesocyclone is an air vortex which is created in a convective storm. The rising air revolves around a vertical axis in a similar direction as the low-pressure systems in the specified hemisphere.

Formation

A fire whirl consists of a burning core and a rotating pocket of air. A fire whirl can reach up to 1,090 °C. Fire whirls occur when a wildfire, or especially firestorm, creates its own wind, which can spawn large vortices. Even bonfires often have whirls on a smaller scale and tiny fire whirls have been generated by very small fires in laboratories.

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 m tall, a few meters wide, and last only a few minutes. Some, however, can be more than 1 km tall, contain wind speeds over 200 km/h (120 mph), and persist for more than 20 minutes.

Fire whirls can uproot trees that are 15 m tall or more. 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.

Fire whirls can be common within the vicinity of a plume during a volcanic eruption. These range from small to large and form from a variety of mechanisms, including those akin to typical firewhirl processes, but can result in cumulonimbus flammagenitus spawning landspouts and waterspouts or even to develop mesoyclone-like updraft rotation of the plume itself and/or of the cumulonimbi, which can spawn tornadoes similar to those in supercells. Pyrocumulonimbi generated by large fires on rare occasions also develops in a similar way.

Classification

Fire whirls are violent and erratic in nature but there are currently three widely recognized types of fire whirls:

Type 1: Stable and centered over burning area.
Type 2: Stable or transient, downwind of burning area.
Type 3: Steady or transient, centered over an open area adjacent to an asymmetric burning area with wind.

The topography and wind patterns influence the type of fire whirl. Understanding of how type 3 fire whirls form is still limited. For example, type 3 fire whirls have been observed to form over water, burning with a blue flame indicating soot free combustion, described as a "blue whirl".

Safety of Aviation

Pilots should be aware of the dangers associated with wildfires. While most commercial flights are only likely to come close to a wildfire on approach to landing, pilots should be aware of the possibility of turbulence and loss of lift when overflying an area of wildfires. On approach, in close proximity to a wildfire, flight crews are likely to experience strong wind shear and reduced visibility. Crews should also keep a lookout for aircraft engaged in fire fighting operations that may be operating in close proximity to controlled airspace and approach lanes.

Related Articles

Further Reading

  • Fire Whirls; Ali Tohidi, Michael J. Gollner, and Huahua Xiao; University of Maryland, Annual Review of Fluid Mechanics 2018 50:1, 187-213.