A type of low pressure system which generally forms in the tropics. It is composed of a large, rotating area of clouds, wind and thunderstorm activity. A tropical cyclone begins as tropical depression, further intensifies to a tropical storm, then becomes a tropical cyclone when its maximum sustained winds reach 74 mph. Depending on its position, a tropical cyclone goes by different names: hurricane in the North Atlantic Ocean, North Pacific Ocean east of the dateline, and the South Pacific Ocean east of 160°E; typhoon in the Northwest Pacific Ocean west of the dateline; severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E; severe cyclonic storm in the North Indian Ocean; tropical cyclone in the Southwest Indian Ocean. These storms can cause immense destruction, but are also an important part of the planet\'s atmospheric circulation.