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A tropical cyclone is a storm system characterized by a low-pressure center and numerous thunderstorms that produce strong winds and flooding rain. [45] A tropical cyclone feeds on heat released when moist air rises, resulting in condensation of water vapour contained in the moist air. [45]
Depth of 26 °C isotherm on October 1, 2006. There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to sustain a low-pressure center, a preexisting low-level focus or disturbance, and low vertical wind shear. [3]
After this process occurs, the growth period of the cyclone, or cyclogenesis, ends, and the low begins to spin down (fill) as more air is converging into the bottom of the cyclone than is being removed out the top since upper-level divergence has decreased. Occasionally, cyclogenesis will re-occur with occluded cyclones.
A tropical cyclone is the generic term for a warm-cored, non-frontal synoptic-scale low-pressure system over tropical or subtropical waters around the world. [4] [5] The systems generally have a well-defined center which is surrounded by deep atmospheric convection and a closed wind circulation at the surface. [4]
In temperate regions, they are most frequent in spring and summer, although they can occur along or ahead of cold fronts at any time of year. [98] They may also occur within a cooler air mass following the passage of a cold front over a relatively warmer body of water. Thunderstorms are rare in polar regions because of cold surface temperatures.
The number of hurricanes is not increasing, but they are becoming more intense as the world warms.
The West Pacific is the most active and the north Indian the least active. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones, super typhoons, or major hurricanes (at least of Category 3 intensity). [1]
The total is likely higher because satellite monitoring technology was not available until the 1960s and cyclones that could have been a Category 5 storm may have remained undetected.