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1 A form of thundery squall encountered in West Africa, particularly around the Gulf of Guinea. It often occurs as a line squall, and is especially frequent at the beginning and end of the rainy season.

2 A violent, rapidly rotating column of air that extends downwards from the base of a cumulonimbus cloud and reaches the ground. Popularly known as a twister. Even if a funnel cloud (tuba) is not visible, the presence of a debris cloud classifies the vortex as a tornado. Tornadoes may develop directly from a rain-free base or from a rotating wall cloud. The most violent tornadoes are produced by supercell thunderstorms, which are the site of strong mesocyclones and the resulting intense updraughts and downdraughts. Extremely strong non-supercell convection may give rise to weaker tornadoes, more properly described as landspouts or waterspouts.

The intensity of tornadoes is generally specified on the Enhanced Fujita Scale (see Fujita scale), based on the damage caused, although in some respects the TORRO scale has advantages, because it is based on wind speed. The record speed for a tornado (512 km h−1) was determined by Doppler radar in the extremely violent tornado that hit the outskirts of Oklahoma City on 3 May 1999. The pressure drop in the centre is estimated to reach 200–250 hPa, which causes the air to reach saturation, giving rise to the visible vortex. Tornadoes have typical durations of 15 minutes, but occasionally much more; diameters of 100–2 000 m; and path-lengths of 10–100 km. Multiple outbreaks frequently occur, one extreme case being on 3–4 April 1974, when 148 were recorded. Some record path-lengths may actually represent a series of tornadoes forming and decaying one after another. See also dry line; feeder band; gustnado; hook echo; suction vortex; undular bore.

Subjects: Meteorology and Climatology.

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