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Tropical Cyclogenesis

written by: futuremet•edited by: Jason C. Chavis•updated: 6/21/2010

Warm sea surface temperatures, low vertical wind shear, and latitudes above ten degrees are the conditions needed for tropical cyclogenesis.

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    Cyclone is the meteorological word for an area of low pressure. Thus, tropical cyclones are “tropical lows." If the tropical low is sufficiently strong, it is called a tropical storm (winds: 39-73 MPH), and if it is very strong, it is called a hurricane (winds greater than 74 MPH). Hurricanes have varying names depending on where on the earth they are located. They are known as typhoons in the west pacific, and cyclones in the Indian Ocean. Regardless, the universal scientific name for these storms is tropical cyclone. Tropical cyclogenesis basically means the formation of tropical cyclones. The prefix “gen" means born or formation, so tropical cyclogenesis reasonably means tropical cyclone formation.

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    How Does a Tropical Cyclone Form?

    Most tropical cyclones form near the Intertropical Convergence Zone (ITCZ)—an area of intense surface convergence near the equator. In a sense, the equator is the ITCZ itself, but since the earth tilts, the area of maximum incoming solar radiation varies during different times of the year. Since the equator is the area of lowest surface pressure, air from both the northern and southern hemispheres converge in this location. The converging air then rises, condenses, and strengthens into vigorous thunderstorms. The ITCZ can be identified as a belt of showers and thunderstorm near the equator in satellite imagery. In the tropical Atlantic, the ITCZ enhances African Easterly Waves (AEWs). These tropical waves travel past the Cape Verde islands and slowly organize to become a tropical depression. A tropical depression has a closed low level circulation with some mid level cyclonic signature. In an ideal environment, a tropical depression will strengthen into a tropical storm—which are tropical cyclones that are often associated with gale force winds. The tropical storm might then strengthen into a Hurricane if the conditions are favorable enough.

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    Locations of Tropical Cyclogenesis

    Tropical cyclones typically form in the tropical regions of the earth, usually between latitudes 10 and 30 (the tropics). Now the question is: why does tropical cyclogenesis most commonly occur at these locations? That is because these regions favor the development of tropical cyclones. The majority of tropical cyclones require sea surface temperatures to be at least 80 degrees Fahrenheit; these temperatures are concealed in the tropics for the majority of the year. Hence, it is usually too cold in the higher latitudes for tropical cyclones to form. However, latitudes 0-10 are also in the tropics; in fact the equator is at latitude 0. Why don’t these regions necessarily favor tropical cyclogenesis? That is because the Coriolis Effect is very weak near the equator and is zero at the equator itself. The Coriolis Effect is what helps the curvature of moving air on earth—it is what helps tropical cyclones rotate. As a result, air tends to travel in straighter lines near the equator.

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    Tropical Cyclone
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    1. University of Wyoming, “Tropical Cyclogenesis" Updated March,2002
    2. University of Pennsylvania Department of Meteorology, William M. Frank, “The Role of Tropical Waves in Tropical Cyclogenesis" Updated September, 2006