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.

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