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Related to Cyclone, Tropical: tropical depression
an atmospheric vortex in the tropical latitudes characterized by low pressure at the center.
Tropical cyclones differ from extratropical cyclones in origin, structure, and development. They are smaller, with diameters on the order of 100–300 km, and the pressure at the center often drops to 950 millibars (1 bar = 105 newtons/m2) and sometimes even below 900 millibars. Consequently, the pressure gradients in a tropical cyclone are very large, and accordingly the wind reaches storm or hurricane force. Therefore, tropical cyclones are subdivided into tropical storms and tropical hurricanes. The winds in tropical cyclones blow counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, moving toward the inside of the cyclone in the lower layers of the atmosphere. In the higher layers, this wind convergence is accompanied by an even stronger divergence. Taken together, this leads to strong ascending air movement throughout the entire tropical cyclone region and to the development of a massive cloud system, with abundant precipitation and thunderstorms. Only a small area within the tropical cyclone, with a radius of 20–50 km (the eye of the storm), is free of a thick cloud cover.
Tropical cyclones originate over the superheated ocean surface in the zone of intertropical convergence if the zone is at least 5° from the equator (so that the deflecting force of the earth’s rotation is sufficiently great). Of the many weak lows that occur in this zone, only about one in ten develops into a violent tropical cyclone. The average number of tropical cyclones each year is only about 80.
The primary source of a tropical cyclone’s energy is the release of enormous quantities of latent heat as the water vapor in the ascending air condenses. Tropical cyclones move slowly, at speeds of 10–20 km/hr, from east to west (in the general direction of air transfer in the tropics), deflecting toward the higher latitudes. Upon reaching land, they rapidly dissipate. Some tropical cyclones move beyond the tropics, turning east as they do so, whereupon they begin to closely resemble extratropical cyclones.
A tropical cyclone exists for periods ranging from a few days to two to three weeks. The high wind velocities, which sometimes reach 70 m/sec with gusts up to 100 m/sec, and the enormous quantities of precipitation, as much as 1,000 mm per day and more, cause sea swells and enormous destruction on land. Flooding occurs in the wake of tropical cyclones not only as a result of precipitation but also as a result of high waves driven by the wind against low-lying shores.
The regions of origin for most tropical cyclones in the northern hemisphere are the Pacific Ocean east of the Philippines, the South China Sea (where they are called typhoons), the Pacific Ocean west of California and Mexico, the Atlantic Ocean east of the Greater Antilles, the Bay of Bengal, and the Arabian Sea. The main areas of origin in the southern hemisphere are the Pacific Ocean east of New Guinea and the Indian Ocean east of Madagascar and northwest of Australia.
REFERENCESRiehl, H. Tropicheskaia meteorologiia. Moscow, 1963. (Translated from English.)
Palmén, E., and C. Newton. Tsirkuliatsionnye sistemy atmosfery. Leningrad, 1973. (Translated from English.)
S. P. KHROMOV