TROPICAL CYCLONE

severe atmospheric disturbance in the tropical oceans between latitudes of approximately 5 and 30 in both hemispheres. These storms are characterized by very low atmospheric pressures in the calm, clear centre of a circular structure of rain, cloud, and very high winds. In the western Atlantic and the Caribbean they are called hurricanes; in the western Pacific, typhoons; and in western Australia, willy-willies (if the surface winds exceed 117 kilometres [73 miles] per hour). Tropical cyclones are cyclonic whirls 80 to 800 kilometres (50 to 500 miles) in diameter; the winds near the centre form an almost circular vortex with a slight inward motion toward the centre near the ocean surface. Because of the Earth's rotation, the vortex circulation is clockwise in the Southern Hemisphere and counterclockwise in the Northern. The areal extent of tropical cyclones is small compared with storms outside the tropics, but the violence of the weather within the disturbed zone is usually far greater. Sustained winds in excess of 160 kilometres (100 miles) per hour are common near the centre, and winds twice that strong have occurred. Very close to the centre of mature cyclones, however, the winds drop abruptly from their extreme maximum to light breezes or even to complete calm. This central circular calm area bears the name eye of the storm and has an average diameter of 24 kilometres (15 miles). The lowest sea-level pressures on Earth occur in or near the eye of a hurricane. All tropical storms develop over water that is warm enough to supply appreciable quantities of vapour to the air. Most storms develop along the equatorial convergence zone, where the trade winds of both hemispheres meet. This zone changes position with the seasons, penetrating to about latitude 15 N between July and October and to latitude 1015 S from January to March or April. These months represent the principal cyclone seasons of the respective hemispheres. The formation of the storms is most frequent farthest from the Equator, where the Coriolis effect (clockwise or counterclockwise rotation due to the rotation of the Earth) is greater (the effect is zero at the Equator). As the warm water evaporates, moist air is carried aloft, where it condenses, releases latent heat, and is warmed further. This warming strengthens the updraft, and a low-pressure area is created in the lower atmosphere. Surrounding air moves into this low-pressure region and feeds the updraft. The Coriolis effect causes the developing storm to spiral, and it continually draws energy from the condensation of lifted water vapour. After several days of intensification, this process produces a mature traveling cyclone. Dissipation of the storms over the tropical oceans is rare. When removed from their oceanic moisture source, however, they lose intensity; even passage over smaller islands of the tropics can result in great reductions in strength. severe atmospheric disturbance in the tropical oceans between latitudes of approximately 5 and 30 in both hemispheres. Such storms are characterized by very low atmospheric pressures in the calm, clear centre of a circular structure of rain, cloud, and very high winds. In the western Atlantic and the Caribbean they are called hurricanes; in the western Pacific, typhoons; and in western Australia, willy-willies (if the surface winds exceed 117 kilometres [73 miles] per hour). Hurricanes and typhoons are characterized by wind speeds exceeding 32 metres per second. The winds spiral inward toward the storm centre (characteristic of cyclonic flow), clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. At the centre of the storm there are light breezes, even the proverbial calm. Barometric pressure decreases rapidly toward the centre, where record low pressures have been recorded. Wind speed, humidity, and rainfall increase toward the central zone, termed the eye, where they suddenly decrease. If there is a strong downdraft in the eye, temperatures there may be some 8 to 10 C higher than in the storm's main body. Such tropical cyclones are not common. Their total number in a given year may vary from 30 to 100, with about one-quarter occurring near Southeast Asia, one-seventh in the Caribbean and adjacent waters, and one-tenth in the southwest Pacific and Australian waters. They play a noticeable, if spasmodic, role in the general circulation of the atmosphere, transporting large amounts of warm, moist air from very low to middle latitudes. It is estimated that a mature hurricane may export more than 3,500,000,000 tons of air per hour, thus contributing greatly to redistribution within the troposphere. The development of a hurricane entails the release of large amounts of energy and the transfer of substantial quantities of water over several degrees of latitude. Joseph Gentilli The Editors of the Encyclopdia Britannica Additional reading Joe R. Eagleman, Severe and Unusual Weather (1983), presents an overview of weather disturbances. Types of storms, their attendant phenomena, and their effects are treated in Richard A. Anthes, Tropical Cyclones: Their Evolution, Structure, and Effects (1982); and Robert H. Simpson and Herbert Riehl, The Hurricane and Its Impact (1981). The Editors of the Encyclopdia Britannica