Cyclone, a system of winds in which air moves around a central area of relatively low pressure. The term cyclone is usually used to refer to large systems of winds that can cover an area 500 miles (800 km) or more in diameter. The term is also sometimes used in the United States to refer to tornadoes.
Because they contain an area of low pressure, cyclones are commonly called lows. They often occur along the boundary between cool and warm air masses in the middle latitudes of the northern and southern hemispheres. A low forms where the air over a large area tends to rise. As the low forms, air from outlying areas begins to flow inward toward the area of low pressure. The earth's rotation causes the winds to follow a curved path. In the Northern Hemisphere, the air in a cyclone circulates in a counterclockwise direction; in the Southern Hemisphere, it circulates in a clockwise direction.
Strong currents of air in the atmosphere, called jet streams, carry cyclones from west to east. Cloudy weather and widespread rain or snow typically occur in a cyclone. As the cyclone approaches a given location, the barometer drops. Once the cyclone has passed by, the barometer begins to rise. A cyclone is generally followed by a high-pressure system, called an anticyclone, or high. In an anticyclone the weather is generally clear and calm. Air in an anticyclone circulates in a clockwise direction in the Northern Hemisphere and in a counterclockwise direction in the Southern.
Tropical cyclones form over warm areas of the Atlantic, Pacific, and Indian oceans. They are carried into middle latitudes by the prevailing winds. Tropical cyclones can develop into large, intense storms with high-speed winds. Depending on where they occur, such storms are known as hurricanes, typhoons, cyclones, and willy-willies.