Atmosphere And Weather

The Life Cycle of a Hurricane

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"The Life Cycle of a Hurricane"
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Each year, far out in the Atlantic Ocean, monsters are born. This region, stretching from the waters off the eastern United States and the Gulf of Mexico southeast to the eastern Atlantic, is where hurricanes begin.

Hurricanes can only form over warm waterat least 27 degrees Centigrade (80 degrees Fahrenheit). These are the basic ingredients of a hurricane: heat and water. As a result, hurricane season runs from June 1 to December 1, the time when the waters of the Atlantic are at heir warmest. Only rarely do hurricanes form at other times of the year.

The first stage of a hurricane's life cycle begins with a tropical disturbance. This is nothing more than thunderstorms forming in a localized area over the ocean. When the tropical disturbance becomes intense enough, it creates a tropical depression (a region of lowered barometric pressure). At this point, winds from surrounding areas of high pressure begin to spiral in toward the center of the developing storm. This spiral is a result of the "Coriolis Effect:" the earth's rotation causes the air currents to follow a curved path. As the spiraling airflow develops, the emerging weather system begins to rotate, and the tropical depression starts to become better organized.

The developing system assumes the shape characteristic of a hurricane, with spiraling rain bands converging on the center, where they merge into an "eyewall" of strong wind and cirrocumulus clouds surrounding a calm eye at the very center. When the winds reach a sustained velocity of 39 miles per hour, the tropical depression is considered a tropical storm. It is at this point in the storm's life that it is given a name by the National Weather Service.

If conditions remain favorable, a tropical storm will continue to draw heat and moisture from the ocean and become larger and more powerful. Within a couple of days (occasionally, as little as half a day) wind speeds reach 74 miles per hour, and the storm becomes a Category 1 hurricane. It may continue to grow stronger, on up to a Category 5 with sustained winds exceeding 155 miles per hour. Hurricanes are slow moving storms, in most cases traveling from 10 to 30 miles per hour. There are rare exceptions, such as the infamous "Long Island Express" that swept up the East Coast in 1938 at speeds of over 60 miles per hour. However fast it travels, though, the hurricane has now become a true monster.

Hurricanes usually last for a week or so, and sometimes two. The longest lived Atlantic hurricane was Ginger, which lasted for 28 days; striking the North Carolina coast in 1971. The longest lived cyclonic storm was Typhoon John, a Pacific storm that lasted for 31 days in 1994.

A hurricane can end in several ways. If it stays out at sea, sooner or later wind sheer will disrupt the pattern of rotating winds that draw moisture and energy from the ocean. Or the storm may wander into colder waters, and so lose its source of energy. For the hurricane, striking a large body of land also spells its end. Once it moves over land, the hurricane no longer has a source of moisture and heat to replenish and power the storm. It weakens to tropical storm strength within hours. As it dissipates, however, the remnants may continue to move inland as torrential rainstorms that can dump 12, 20, or more inches of rain over areas stretching for hundreds of miles inland, adding to the destruction caused when the hurricane came ashore.

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