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A tropical cyclone is a storm system characterized by a low-pressure center and a spiral arrangement of thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when evaporating water from the ocean is released as saturated air rises, resulting in the condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows. The characteristic that separates tropical cyclones from these other systems is that, at any height in the atmosphere, the center of a tropical cyclone will be warmer than its surroundings—a phenomenon called "warm core" storm systems. The term "tropical" refers both to the geographical origin of these systems, which usually form in tropical regions of the globe, and to their formation in maritime tropical air masses. The term "cyclone" refers to their cyclonic nature, with counterclockwise wind flow in the Northern Hemisphere and clockwise flow in the Southern Hemisphere. The opposite direction of the wind flow is due to the Coriolis force. Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. (via Freebase)