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 (/ˈhʌrɨkeɪn/, /ˈhʌrɨkən/), typhoon (/taɪ'fuːn/), tropical storm, cyclonic storm, tropical depression, and simply cyclone. While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to generate high waves, damaging storm surge, and tornadoes. They develop over large bodies of warm water, and lose their strength if they move over land because of increased surface friction and loss of the warm ocean as an energy source. This is why coastal regions can sustain significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can cause significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. While their effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain relatively stable and warm temperatures worldwide and help to maintain equilibrium in the Earth's troposphere. Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. The background environment is modulated by climatological cycles and patterns such as the Madden-Julian oscillation, El Niño-Southern Oscillation, and the Atlantic multidecadal oscillation. Others form when other types of cyclones acquire tropical characteristics. Tropical cyclones are moved by steering winds in the troposphere; if the conditions remain favorable, the systems intensify and can even develop an eye. Concurrently, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. It is not possible to artificially induce the dissipation of these systems with current technology.

• published: 19 Nov 2013
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