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How Hurricanes Form - Windows to the Universe

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How Hurricanes Form

A thunderstorm in the tropics can grow into a massive hurricane under certain conditions. Sometimes several tropical thunderstorms are able to organize, rotating around a central area of low pressure. This is called a tropical depression. If the depression strengthens so that winds reach at least 39 mph, it is called a tropical storm. And if wind speeds increase to more than 74 mph, it is called a tropical cyclone or hurricane.

A study found that out of 608 possible storms only 50 developed into tropical storms. A storm will strengthen if there is a supply of warm, moist air to feed it. Warm, moist air is found above warm, tropical ocean waters.

Once they form, hurricanes take energy from the warm ocean water to become stronger. While a hurricane is over warm water it will continue to grow. A hurricane dies when it moves away from the tropics. When a hurricane moves into higher latitudes where there is cooler ocean water, it looses its power. There is no longer a supply of warm, moist air near the ocean surface to feed the storm so it shrinks. It will also weaken if it travels over land.

The rotation of the storm is due to the Coriolis Effect, a product of the Earth's rotation. This causes the air being drawn into the central low pressure to curve. The incoming air must go somewhere so it rises as it rotates. This rising air, which is saturated with water, cools and condenses, forming clouds. Hurricanes do not occur within 300 miles (500 kilometers) of the equator because there is no Coriolis Effect at the equator.

Last modified March 13, 2009 by Lisa Gardiner.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF