Tropical cyclones are the most powerful storms on Earth. Referred to by different names in different regions of the world, they form over warm, tropical waters. In India and Australia, they are called cyclones; in the western North Pacific and the Philippines, they are called typhoons; and in the Atlantic, they are called hurricanes. Regardless of the name they go by, these storms are defined by their size and intensity and by the weather patterns that drive them. They produce sustained winds of 120 to more than 240 kilometers per hour (74 to more than 150 miles per hour), can dump more than 2.5 centimeters (1 inch) of rain per hour, and can trigger sudden, unpredictable surges in ocean tides, all of which can cause extensive property damage and loss of life. Hurricane Andrew, which struck southeast Florida in 1992, was responsible for at least 50 deaths and more than $26 billion in property damage.

Hurricanes originate over warm tropical oceans. In fact, the heat of the water provides the energy needed for the storm to develop and maintain itself. Water at temperatures below 26°C (80°F) does not possess sufficient heat to generate and sustain a hurricane. This is why hurricanes in the northern hemisphere develop in the late summer or early autumn months, when Caribbean and Atlantic Ocean temperatures are at their warmest.

A hurricane begins when winds blowing across the surface of the warm ocean water converge to form clusters of strong thunderstorms known as tropical disturbances or tropical waves. The water fuels these thunderstorms, causing them to build. As they do, more water evaporates from the ocean surface and enters the air as water vapor. When the rising air cools, the water vapor condenses to form clouds, a process that releases heat. This heat warms the center, or eye, of the storm, which in turn causes more water to evaporate. Meteorologists call this chain reaction a "heat engine." The result is a reduction of air pressure at the water's surface and — if the winds, moisture, and ocean temperature continue to remain favorable — the formation of a hurricane.

There is a limit to the size and intensity a hurricane can reach, and clearly none of these storms lasts forever. One of the longest lasting was the North Pacific Hurricane Tina, which maintained hurricane force winds for 24 days and traveled thousands of kilometers before it dissipated. Most hurricanes survive less than a week. When they pass over cooler ocean waters or over land, they lose contact with the heat energy that sustains them and dissipate.

Hurricanes are classified into 5 categories, based on their size and intensity. Category 1 hurricanes are the smallest and category 5 hurricanes are the largest. Categories are based on wind speed and the potential for damage.


Source: Teachers' Domain, How Hurricanes Form, published December 17, 2005, retrieved on December 2, 2010, http://www.teachersdomain.org/resource/ess05.sci.ess.watcyc.hurrlife/