cyclone (redirected from Cyclones)

cyclone, atmospheric pressure distribution in which there is a low central pressure relative to the surrounding pressure. The resulting pressure gradient, combined with the Coriolis effect, causes air to circulate about the core of lowest pressure in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Near the surface of the earth, the frictional drag on the air moving over land or water causes it to spiral gradually inward toward lower pressures. This inward movement of air is compensated for by rising currents near the center, which are cooled by expansion when they reach the lower pressures of higher altitudes. The cooling, in turn, greatly increases the relative humidity of the air, so that "lows" are generally characterized by cloudiness and high humidity; they are thus often referred to simply as storms.

According to the theory first proposed by the Norwegian physicist Vilhelm Bjerknes, the extratropical, or middle-latitude, cyclone originates as a wave, or perturbation, in the polar front separating the cold polar easterly winds from the warmer prevailing winds farther toward the equator. This wave, once induced by the opposing air currents, is accentuated by the rotational sense of the circulation, which pumps warm, moist air toward the pole around the eastern side of the cyclone center and cold, dry air toward the equator to the west of the center. Such wave cyclones often intensify, expanding the radius of the affected area to 500 mi (805 km) or more, while reducing atmospheric pressure, especially toward the center.

Tropical cyclones, formed over warm tropical oceans, are not associated with fronts, as are the middle-latitude wave cyclones, nor are they as large as the latter. A tropical cyclone that has matured to a severe intensity is called a hurricane when it occurs in the Atlantic Ocean or adjacent seas, a typhoon when it occurs in the Pacific Ocean or adjacent seas, or simply a cyclone or tropical cyclone when it occurs in the Indian Ocean region.

Cyclones in middle latitudes move generally from west to east along with the prevailing winds and cover 500 to 1,000 mi (800–1,610 km) each day; tropical cyclones usually move toward the west with the flow of the trade winds during their formative stages, then curve toward the poles around subtropical anticyclones.

Bibliography

See D. Longshore, Encyclopedia of Hurricanes, Typhoons, and Cyclones (1998).

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cyclone

Any large system of winds that circulates about a centre of low atmospheric pressure in a counterclockwise direction north of the Equator and in a clockwise direction south of it. Cyclones that occur in the mid- and high latitudes are known as extratropical cyclones; they are frequently preceded by thickening and lowering clouds, followed by precipitation. Cyclones that form in the lower latitudes are known as tropical cyclones; smaller than extratropical cyclones, they tend to be more violent and can cause considerable damage (see tropical cyclone). Wind systems that circulate around a high-pressure centre in directions opposite to that of cyclones are known as anticyclones.

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cyclone

1. another name for depression

2. a violent tropical storm; hurricane

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cyclone [′sī‚klōn]

(chemical engineering)

A static reaction vessel in which fluids under pressure form a vortex.

(mechanical engineering)

Any cone-shaped air-cleaning apparatus operated by centrifugal separation that is used in particle collecting and fine grinding operations.

(meteorology)

A low-pressure region of the earth's atmosphere with roundish to elongated-oval ground plan, in-moving air currents, centrally upward air movement, and generally outward movement at various higher elevations in the troposphere.