A satellite image of a swirling hurricane Men with gas masks and protective suits on The planetary rover Sojourner A large group of refugees huddled together







Hurricanes have been likened to machines that convert the warmth of tropical oceans into wind and waves. The essential fuels are heat and moisture.

In the Atlantic, their way is prepared by the huge carousel of air that is the Bermuda high-pressure system, also known as the Azores High. This movement causes the easterly trade winds, which can pick up atmospheric disturbances from as far away as West Africa. Some of the concentrated showers that ride these trades mature into thunderstorms. Atmospheric pressure at the surface drops as the warm, moist air of summer rises, creating a vacuum. Surrounding air rushes in to fill the void. The final touch is provided by the Earth’s rotation, which sets the whole thing spinning.

Sometimes the cycle perpetuates itself, drawing in more air, spinning faster, growing ever larger. If sustained winds reach 39 miles (63 km) an hour, the depression becomes a tropical storm and acquires a name. At 74 miles (119 km) an hour, it reaches a whole new level of respect and attention: It becomes a hurricane.

Storm surge results from low air pressure at the center of the maelstrom, which causes the ocean to rise three or more feet (one meter) higher than the surrounding water. More water, driven by wind, piles onto this bulge as the system approaches land. The slope of a shoreline can cause the onrushing seas to rise as high as 25 feet (8 meters) above normal tide. These factors, along with storm-driven waves, can overwhelm miles of shoreline with devastating force.

A satellite image of five hurricane storms moving eastward   Click on this photo to enter photo gallery

Watch an animation showing how hurricanes form.

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Hurricanes range in size from 50 to 500 miles (80 to 800 kilometers) in diameter.

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