Tuesday, May 6, 2008

What is the Difference Between a Hurricane, Cyclone, and Typhoon?

Hurricanes, typhoons and tropical cyclones are different names for the same type of storm. A tropical cyclone is called a hurricane in the North Atlantic Ocean, South Pacific Ocean, or the Northeast Pacific Ocean on the eastern side of the dateline. A typhoon occurs in the Northwest Pacific Ocean west of the dateline. In other parts of the world, these storms are called severe tropical cyclones.

A hurricane, then, is a cyclonic storm with maximum sustained winds over 74 mph (64 knots; 119 kph). Hurricanes are then further classed according to strength by the Saffir-Simpson Scale. There are five strength categories, with a Category 5 storm rating maximum sustained winds over 156 mph (136 knots; 251 kph).

A hurricane generally starts as an organized band of convection, or thunderstorms, called a tropical wave. When conditions are favorable, the wave starts to further organize and strengthen. Convection increases and the wave starts taking on cyclonic characteristics. If strengthening continues, it develops an eye and eyewall and soon becomes a hurricane. Favorable conditions for hurricane development include the system being over very warm water, and in an environment with little wind shear. Wind shear “tears up” a hurricane because it sends winds in the opposite direction, thereby inhibiting the cyclone’s formation.

A cyclone itself is often a generic name for any kind of violent windstorm, and particularly in the Midwestern United States, is the name for a tornado. However, a tornado and a hurricane are two entirely different storms. A tornado is usually the result of a mesocyclone, or severe thunderstorm, over land, although, strangely enough, a landfalling hurricane can spawn tornadoes. A tornado is also a smaller, short-lived storm, while a hurricane covers several hundred square miles or kilometers and may last for several days over water.

The landfalling hurricane quickly loses strength because it is deprived of the heat and moisture from the ocean water keeping it alive. A hurricane that passes over land and goes back into the ocean, however, may regenerate.

Although most people think of a hurricane as being primarily a wind storm, its real damage is usually caused by flooding. This was evident in 2005, when Hurricane Katrina struck the Louisiana and Mississippi Gulf Coasts. The storm had dropped to a Category 3 by landfall, but the storm surge in front of it was driven by the storm when it was a Category 5. The storm surge was well over 20 feet in some areas, and wiped out thousands of homes in its path.

The National Weather Service’s Tropical Prediction Center tracks storms in the North and East Atlantic, and in the Eastern Pacific Oceans. Their Web site provides a wealth of information on the formation, tracking and forecasting hurricanes, as well as U.S. hurricane statistics over the years.

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