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.
See D. Longshore,
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