Science Dictionary
cyclone   (sī'klōn')  Pronunciation Key 
  1. A large-scale system of winds that spiral in toward a region of low atmospheric pressure. A cyclone's rotational direction is opposite to that of an anticyclone. In the Northern hemisphere, a cyclone rotates counterclockwise; in the Southern hemisphere, clockwise. Because low-pressure systems generally produce clouds and precipitation, cyclones are often simply referred to as storms. ◇ An extratropical cyclone is one that forms outside the tropics at middle or high latitudes. Extratropical cyclones usually have an organized front and migrate eastward with the prevailing westerly winds of those latitudes. ◇ A tropical cyclone forms over warm tropical waters and is generally smaller than an extratropical cyclone. Such a system is characterized by a warm, well-defined core and can range in intensity from a tropical depression to a hurricane. Compare anticyclone.

  2. A small-scale, violently rotating windstorm, such as a tornado or waterspout. Not in scientific use.

Our Living Language  : Technically, a cyclone is nothing more than a region of low pressure around which air flows in an inward spiral. In the Northern Hemisphere the air moves counterclockwise around the low-pressure center, and in the Southern Hemisphere the air travels clockwise. Meteorologists also refer to tropical cyclones, which are cyclonic low-pressure systems that develop over warm water. For a tropical cyclone to originate, a large area of ocean must have a surface temperature greater than 27 degrees Celsius (80.6 degrees Fahrenheit). Tropical cyclones are categorized based on the strength of their sustained surface winds. They may begin as a tropical depression, with winds less than 39 miles (63 kilometers) per hour. Tropical storms are identified and tracked once the winds exceed this speed. Severe tropical cyclones, with winds of 74 miles (119 kilometers) per hour or greater, are better known as hurricanes when they occur in the Atlantic Ocean and Gulf of Mexico, or as typhoons when they happen in the Pacific Ocean. Because the word cyclone broadly defines a kind of air flow, cyclones are not confined to our planet. In 1999 the Hubble Space Telescope photographed a cyclone more than 1,610 kilometers (1,000 miles) across in the northern polar regions of Mars.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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