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An atmospheric vortex associated with the Gust Front ahead of a severe Cumulonimbus (Cb) (Thunderstorm) cell.
Strong downdrafts of rain/Hail cooled air from the advancing edge of a Thunderstorm create strong, straight line winds ahead of the storm. The leading edge of this mass of cooled air, where it meets warmer surface air, is called the Gust Front. The gust front may be 2 or 3 nautical miles ahead of the storm.
If the wind speed is in excess of 60 kts, then surface friction, which disturbs the straight line flow, can cause the formation of a spinning vortex at the Gust Front. The vortex starts from the ground upwards and can reach 300 feet in height, normally made visible by dust and debris. This vortex is known as a Gustnado.
Although the name is derived from a combination of "Gust Front" and "Tornado", a Gustnado is not a Tornado. A Tornado is associated with the warm and powerful updrafts which feed the cloud, forms from and is connected to the base of the Cumulonimbus cloud, and the rotation is driven by the mesocyclonic rotation of the cloud itself. A gustnado is not connected to the cloud base, is associated with the cool downdrafts ahead of, and occasionally behind, the storm, and are shortlived in duration.
It is rare for an aircraft to encounter a Gustnado in flight because of the shortlived and shallow nature of the phenomenon, and also because pilots are wary of the greater dangers associated with microbursts and cumulonimbus clouds and so stay well clear of such storms. An aircraft is most likely to encounter a Gustnado on take-off or landing.
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