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The most destructive of all weather phenomena is the tropical cyclone that reaches hurricane proportions. While a tornado exceeds the severity of a full-fledged tropical cyclone in a smaller area, it has a comparatively short life cycle and a much smaller path of destruction. The tropical cyclone, because of its greater horizontal extent and longer life, exceeds any other phenomenon in total damage and loss of life. Tropical cyclones are given various names in different regions of the world, but they all have essentially the same characteristics. They have no distinct cold and warm sectors, no well defined surfaces of discontinuity or fronts at the surface, and no migratory high-pressure systems ac-companying them until they leave the tropics.

They develop over tropical or subtropical waters and have definite, organized circulations. In midlatitudes, tropical cyclones take on some of the characteristics of extratropical cyclones. Some distinctive characteristics are the region of calm or relative calm winds, called the "eye"; its westward movement (prior to coming under the influence of the midlatitude westerlies); and the distribution of rainfall. While extratropical cyclones are more frequent in winter, tropical cyclones occur most frequently in" summer and autumn. They are roughly circular or elliptical in shape and are small in comparison to extratropical systems.

Classification of Tropical Cyclones

There are three recognized categories of tropical cyclones, all of which must show evidence of a closed circulation at the surface. The three classes are based on observed or estimated surface wind speeds. Their definitions are as follows:

TROPICAL DEPRESSION—A tropical cyclone whose sustained winds are less than 34 knots and whose pressure pattern exhibits one or more closed isobars on the surface

TROPICAL STORM—A tropical cyclone whose pressure pattern results in sustained surface winds of 34 to 63 knots, inclusive

HURRICANE OR TYPHOON—A tropical cyclone whose sustained surface winds exceed 63 knots

Life Cycle

Tropical cyclones are sustained by the energy released through the latent heat of condensation. This energy is furnished to tropical cyclones by the warm waters over which they develop and move. The warm moist air is lifted by a combination of convergence and instability, and upon condensing, the latent heat is liberated. When these systems move over land, the energy source is cut off, and they eventually dissipate. Tropical cyclones also weaken on moving over the colder waters of higher latitudes. Any decrease in the moisture and heat supplied by warm ocean waters weakens them.

The average life-span of tropical cyclones is about 6 days from the time they form until they either move over land or recurve to higher latitudes. Some last only a few hours, while others last as long as 2 weeks. The life cycle of the average tropical cyclone is divided into four stages.

FORMATIVE OR INCIPIENT STAGE.— This stage starts with the birth of the cyclonic circulation and ends at the time hurricane intensity is reached. This stage can be slow, requiring days for a weak circulation to begin, or in the case of an easterly wave, it can be relatively explosive, producing a well-formed eye in as little as 12 hours. In this stage, the minimum pressure reached is about 975 mb. A good indication that a system of this type has formed or is forming is the appearance of westerly winds (usually 10 knots or more) in those tropical latitudes where easterly winds normally prevail.

IMMATURE OR INTENSIFICATION STAGE.— This stage lasts from the time the system reaches hurricane or typhoon intensity until the time it reaches its maximum strength and lowest central pressure. The lowest central pressure often drops well below 975 mb, and the winds spiral in a tight ring around the eye with a fair degree of symmetry. Usually, the radius of strongest winds are no more than 60 miles around the center. This development may take place gradually or occur in less than a day. The cloud and precipitation fields develop into narrow, inward-spiraling bands.

MATURE STAGE.— This stage lasts from the time a typhoon or hurricane attains its maximum intensity until it weakens to below this intensity or transforms, and is reclassified, as an extra-tropical cyclone. In this stage, the typhoon or hurricane may exist for several days at nearly the same intensity level, or decrease slowly. Strong winds extend farther and farther from the center, affecting a much greater area. The winds and weather usually extend out farther in the right semicircle. By the time this stage is reached, the tropical cyclone is usually well advanced toward the north and west, or it has recurved under the influence of the midlatitude westerlies. The typhoons of the Pacific usually last longer in the mature stage and grow to larger sizes than the hurricanes of the Atlantic.

DECAYING STAGE.— This stage is charac-terized by rapid decay, as in the case of many tropical cyclones that move inland, or as associ-ated with recurvature and transformation into an extratropical cyclone. In the case of a tropical cyclone moving over land, it steadily loses its strength and character. In regard to transforma-tion, regeneration frequently occurs and results in maintenance or redevelopment of strong winds and other hurricane or typhoon characteristics. There is no set duration for the time a tropical cyclone may be in any one stage. It is entirely possible that a stage may be skipped or gone through in such a short time that it may be missed without synoptic data. Satellites permit almost constant viewing of tropical systems and have led to a classification system based on the appearance of the cloud system associated with tropical cyclones. We will discuss this system in unit 10.

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