Tropical Cyclone
Tropical Cyclone

Tropical cyclones form when warm ocean water evaporates, is further warmed by the sun, and rises to create a high, thick layer of humid air. This rising of warm, dense air creates an area of low pressure, technically known as a depression, near the ocean's surface. Surface winds converge and, due to the earth's Coriolis force, display a clear cyclonic pattern.

The inward rush of peripheral surface winds toward the central area of low pressure, the rise of warm humid air in the center, and the subsequent outflow away from the system at high altitude, combine to create a self-sustaining heat engine. The warmer the water temperature, the faster the air in the center of the system rises. The faster this air rises, the greater will be the difference between the surface air pressures inside and outside the vortex.

Air flows from areas of relative high pressure to relative low pressure. The greater the difference between peripheral and central pressures, the faster the inflow. When wind speeds reach 40 miles per hour, the depression reaches tropical storm status. When wind speeds reach 74 miles per hour, the storm is designated a hurricane or typhoon. The term "super-typhoon" is used for tropical cyclones that reach maximum sustained 1-minute surface winds of at least 130 knots, which is the equivalent of a strong Category 4 or Category 5 hurricane in the Atlantic basin. Note that the terms "hurricane" and "typhoon" are regionally specific names for the same phenomenon. Severe tropical cyclones that occur in the Atlantic and eastern Pacific are referred to as hurricanes, and in the western Pacific as typhoons.

The intensity of tropical cyclones varies by ocean basin. Of the approximately 80 tropical cyclones that form each year throughout the world, more than half will develop into severe storms with wind speeds greater than 74 miles per hour. In the western North Pacific, however, this figure is significantly higher. Over two-thirds of all tropical cyclones that form in this region are likely to develop into typhoons. The primary reason for this region's propensity to spawn intense storms is the extremely large area of very warm water. It is not unusual for water temperatures here to exceed 86 degrees Fahrenheit. Another reason is the formation of seasonal monsoon troughs, which are climatologically preferred for tropical cyclone formation. In fact, the lack of a monsoon trough in the Atlantic may be one reason why tropical cyclones form less frequently there and are typically less intense.


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