CYCLONE

any large system of winds that rotates about a centre of low atmospheric pressure in a counterclockwise direction north of the Equator and in a clockwise direction to the south. Cyclonic winds move across nearly all regions of the Earth except the equatorial belt and are generally associated with rain or snow. Also occurring in much the same areas are anticyclones, wind systems that rotate about a high-pressure centre. Anticyclones are so called because they have a flow opposite to that of cyclonesi.e., an outward-spiralling motion, with the winds rotating clockwise in the Northern Hemisphere and counterclockwise in the Southern. These winds are usually not as strong as the cyclonic variety and bring no precipitation. Cyclones occur chiefly in the midlatitude belts of both hemispheres. In the Southern Hemisphere, where most of the terrestrial surface is covered by the oceans, cyclones are distributed in a relatively uniform manner through various longitudes. Characteristically, they form in latitudes 30 to 40 S and move in a generally southeasterly direction, reaching maturity in latitudes around 60. The situation is quite different in the Northern Hemisphere. There, continental landmasses extend from the Equator to the Arctic, and large mountain belts interfere with the midlatitude air currents, giving rise to significant variations in the occurrence of cyclones (and anticyclones). Certain tracks are favoured by the wind systems. The principal cyclone tracks lie over the oceans, the ocean surface being smoother than that of the land and thus offering less resistance to the strong winds around low-pressure centres. Cyclones that form closer to the Equator (i.e., in latitude 10 to 15 north and south over the oceans) differ somewhat in character from the extratropical variety. Such wind systems, known as tropical cyclones, are much smaller in diameter. Whereas extratropical cyclones range from nearly 1,000 to 4,000 kilometres (620 to 2,500 miles) across, tropical cyclones measure only about 100 to 500 kilometres in diameter. They also tend to be more violent than those occurring in the midlatitudes and can cause considerable damage. Their wind velocities may reach up to 60 metres per second (nearly 200 feet per second), as opposed to about 30 metres per second for extratropical cyclones. In the Atlantic and Caribbean regions, tropical cyclones are commonly called hurricanes, while in the western Pacific and China Sea the term typhoon is applied.