Friday, August 26, 2005

The nature and history of hurricanes

In meteorology, a tropical cyclone (or tropical storm, typhoon, or hurricane, depending on strength and location) is a type of low pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.
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Structurally, a tropical cyclone is a large, rotating system of clouds, wind and thunderstorm activity. The primary energy source of a tropical cyclone is the release of the heat of condensation from water vapor condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.
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The ingredients for a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.
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This use of condensation as a driving force is the primary difference which distinguishes tropical cyclones from other meteorological phenomena. Mid-latitude cyclones, for example, draw their energy mostly from pre-existing temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength will diminish rapidly.
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Tropical cyclones can occasionally form despite not meeting these conditions. A combination of a pre-existing disturbance, upper level divergence and a monsoon-related cold spell led to the creation of Typhoon Vamei at only 1.5 degrees north of the equator in 2001. It is estimated that the factors leading to the formation of this typhoon occur only once every 400 years.
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Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. However, each particular basin has its own seasonal patterns.
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In the North Atlantic, a distinct hurricane season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the North Atlantic hurricane season is September 10. The Northeast Pacific has a broader period of activity, but in a similar timeframe to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November.
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In the Southern Hemisphere, tropical cyclone activity begins in late October and ends in May. Southern Hemisphere activity peaks in mid-February to early March.
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Nearly all tropical cyclones form within 30 degrees of the equator and 87% form within 20 degrees of it. However, because the Coriolis effect initiates and maintains tropical cyclone rotation, such cyclones almost never form or move within about 10 degrees of the equator [1] (where the Coriolis effect is weakest). However, it is possible for tropical cyclones to form within this boundary if another source of initial rotation is provided. These conditions are extremely rare, and such storms are believed to form at a rate of less than one a century.
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Most tropical cyclones form in a worldwide band of thunderstorm activity known as the Intertropical convergence zone (ITCZ).
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Worldwide, an average of 80 tropical cyclones form each year.
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Tropical cyclones are classified into three main groups: tropical depressions, tropical storms, and a third group whose name depends on the region.
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A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 metres per second (33 knots, 38 mph, or 62 km/h). It has no eye, and does not typically have the spiral shape of more powerful storms. It is already becoming a low-pressure system, however, hence the name "depression".
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A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 meters per second (34�63 knots, 39�73 mph, or 62�117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present. Government weather services assign first names to systems that reach this intensity (thus the term named storm).
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At hurricane intensity, a tropical cyclone tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of the circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 10 to 50 miles (16 to 80 kilometers) wide in which the strongest thunderstorms and winds circulate around the storm's center.
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The circulation of clouds around a cyclone's center imparts a distinct spiral shape to the system. Bands or arms may extend over great distances as clouds are drawn toward the cyclone. The direction of the cyclonic circulation depends on the hemisphere; it is counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. Maximum sustained winds in the strongest tropical cyclones have been measured at more than 85 m/s (165 knots, 190 mph, 305 km/h). Intense, mature hurricanes can sometimes exhibit an inward curving of the eyewall top that resembles a football stadium: this amazing phenomenon is thus sometimes referred to as stadium effect.
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While the most obvious motion of clouds is toward the center, tropical cyclones also develop an outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through a chimney effect of the storm engine. This outflow produces high, thin cirrus clouds that spiral away from the center. The high cirrus clouds may be the first signs of an approaching hurricane.