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Whirlwind, any rotating air mass, including the tornado and the large cyclonic and anticyclonic storm, or cyclone. In meteorology, the term whirlwind is more strictly applied to the smaller swirling atmospheric phenomenon commonly known as dust devil or dust whirl, which occurs mostly over deserts and semi-arid plains during hot, calm days.

The principal cause of whirlwinds is intense insolation, or incoming solar radiation received by the Earth, which produces an overheated air mass just above the ground. This air mass rises, usually in the form of a cylindrical column, sucking up loose surface material, such as dust, sand, and leaves. Whirlwinds vary in height from 30 to 152 m (100 to 300 ft), but exceptionally vigorous dust devils may exceed 1,524 m (5,000 ft) in height. The vortices of whirlwinds range in size from a few metres to several hundred metres and, depending on their force and size, dust devils may disappear in seconds or last several hours. Brief whirlwinds are erratic in motion, but the longer-lasting ones move slowly with the prevailing winds.

Water whirlwinds, commonly called waterspouts, are whirling columns of air and watery mist. The mist is mainly fresh water, formed by condensation in the atmosphere. Water whirlwinds are frequent occurrences over oceans and lakes but are seldom violent. Convective storms generate most waterspouts, and the rare tornadic spouts are generated in thunderstorms, in association with tropical cyclones or cold frontal squalls. Fire and smoke whirlwinds are caused by forest, oil, and incendiary-bomb fires, and they may have large, violent vortices.

Dust and fire whirlwinds are reported in the Old Testament, notably in the Book of Job. Aristotle attempted to explain whirlwinds in his Meteorologica, a study of meteorological phenomena.

Tornado Watching

A meteorologist tracks a tornado as part of continuing weather observations to learn more about the earth’s atmosphere. Since the 19th century, scientific forecasting has greatly improved. Weather radar can detect and track tornados, hurricanes, and other severe storms.

Tornado (Latin, tonare, “to thunder”), in meteorology, violent whirling wind, characteristically accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as a cyclone or twister, a tornado can be a few metres to about a kilometre wide where it touches the ground, with an average width of a few hundred metres. It can move over land for distances ranging from short hops to many kilometres, causing great damage wherever it descends. The funnel is made visible by the dust that is sucked up and by condensation of water droplets in the centre of the funnel. The same condensation process makes visible the generally weaker sea-going tornadoes, called waterspouts, that occur most frequently in tropical waters. Most tornadoes spin anticlockwise in the northern hemisphere and clockwise in the southern, but occasional tornadoes reverse this.

The exact mechanisms that cause a tornado to form are still not fully understood, but the funnels are always associated with violent motions in the atmosphere, including strong updraughts and the passage of fronts. They develop within low-pressure areas of high winds; the speed of the funnel winds themselves is often placed at more than 480 km/hr (300 mph), although speeds of more than 800 km/hr (500 mph) have been estimated for extremely strong storms. Damage to property hit by a tornado results both from these winds and from the extremely reduced pressure in the centre of the funnel, which causes structures to explode when they are not sufficiently ventilated to adjust rapidly to the pressure difference. The pressure reduction is in keeping with Bernoulli_s principle, which states that pressure is reduced as velocity increases.

Tornadoes are most common and strongest in temperate latitudes. In the United States they tend to form most frequently in the early spring; the “tornado season” shifts towards later months with increasing latitude. The number of funnels observed each year can vary greatly in any given region.

Hurricane, name applied to migratory tropical cyclones that originate over oceans in certain regions near the equator, and particularly to those arising in the West Indian region, including the Caribbean Sea and the Gulf of Mexico. Hurricane-type cyclones in the western Pacific are known as typhoons.

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