Cyclone is often used to mean a violent, swirling windstorm. To scientists, however, the term cyclone more commonly refers to the weather system in which this type of storm occurs. In this sense, a cyclone is any weather system except a tornado in which the air pressure at the earth’s surface is relatively low.
A cyclone—in the sense of a low-pressure system—acts as a “weather maker” in the middle latitudes. These are zones that extend from the Tropic of Cancer to the Arctic Circle and from the Tropic of Capricorn to the Antarctic Circle. A cyclone may be accompanied by strong winds and widespread areas of cloudiness and precipitation. A single cyclone can affect the weather over a third of a continent or more.
Viewed from above, the surface winds of a cyclone in the Northern Hemisphere blow counterclockwise and inward. In the Southern Hemisphere, the winds blow clockwise and inward. The winds bring together contrasting masses of air in the middle latitudes and in the high latitudes north of the Arctic Circle and south of the Antarctic Circle. Air masses may differ in temperature, humidity, or both. Where contrasting air masses meet, warm and cold fronts develop and spiral outward from the cyclone center. Warm air rises along the fronts, often producing cloudiness and rain or snow. Prevailing westerlies (winds from the west) steer a middle-latitude cyclone to the east and northeast.
A different kind of cyclone develops in or near the tropics, the regions between the equator and the Tropics of Cancer and Capricorn. A tropical cyclone has about one-third the diameter of a middle-latitude cyclone; it forms in warm, humid air over very warm ocean water; and it has no fronts. Prevailing winds steer it to the west.
The most intense tropical cyclone is a storm with extremely low air pressure at its center, surface winds blowing at speeds greater than 74 miles (119 kilometers) per hour, and heavy rains. Such a storm is called a hurricane when it occurs over the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Northeast Pacific Ocean. In other parts of the world, it is known as either a typhoon, severe tropical cyclone, severe cyclonic storm, or simply tropical cyclone.
A swirling storm that develops from a rapid drop in atmospheric pressure is sometimes called a bomb cyclone. Atmospheric pressure must drop by 24 millibars of mercury or more as measured in a barometer within 24 hours for a storm to be called a bomb cyclone. Such drops in pressure occur periodically in the atmosphere when a mass of very cold air meets a mass of warmer air. The formation of such a storm is called bombogenesis.
See also Hurricane; Tornado; Typhoon; Weather; Wind.
