Kategorie: Angličtina (celkem: 879 referátů a seminárek)

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  • Přidal/a: anonymous
  • Datum přidání: 23. září 2006
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Hurricanes: The Greatest Storms on Earth
Few things in nature can compare to the destructive force of a hurricane. Called the greatest storm on Earth, a hurricane is capable of annihilating coastal areas with sustained winds of 155 mph or higher and intense areas of rainfall and a storm surge. In fact, during its life cycle a hurricane can expend as much energy as 10,000 nuclear bombs!
The term hurricane is derived from Huracan, a god of evil recognized by the Tainos, an ancient aborigines Central American tribe. In other parts of the world, hurricanes are known by different names. In the western Pacific and China Sea area, hurricanes are known as typhoons, from the Cantonese tai-fung, meaning great wind. In Bangladesh, Pakistan, India, and Australia, they are known as cyclones, and finally, in the Philippines, they are known as baguios. Hurricane Formation and Decay
Hurricanes form over tropical waters (between 8° and 20° latitude) in areas of high humidity, light winds, and warm sea surface temperatures (typically 26.5°C [80°F] or greater). These conditions usually prevail in the summer and early fall months of the tropical North Atlantic and North Pacific Oceans and for this reason, hurricane "season" in the northern hemisphere runs from June through November.
The first sign of hurricane genesis (development) is the appearance of a cluster of thunderstorms over the tropical oceans, called a tropical disturbance. Tropical disturbances most commonly form in one of three different ways, all of which involve the convergence of surface winds. Near the equator, the easterly trade winds converge (come together) to trigger numerous thunderstorms in a region called the Intertropical Convergence Zone (ITCZ). Occasionally, a cluster of thunderstorms will break away from the ITCZ and become better organized. Another mechanism is the convergence of air that occurs along a mid-latitude frontal boundary that has made its way into the Gulf of Mexico or off the East Coast of Florida. The last mechanism is the easterly wave, a tropical disturbance that travels from east to west in the region of the tropical easterlies. Converging winds on the east side of the easterly wave trigger the development of thunderstorms.

In fact, most Atlantic hurricanes can be traced to easterly waves that form over Western Africa.
Given favorable conditions, the tropical disturbance can become better organized, indicated by falling surface pressures in the area around the storm and the development of a cyclonic circulation (counter-clockwise in the Northern Hemisphere). Surface pressures fall as water vapor condenses and releases latent heat into areas within the tropical disturbance. (Latent heat is the heat energy released or absorbed during the phase change of a substance—in this case, water vapor.) In response to the atmospheric heating, the surrounding air becomes less dense and begins to rise. As the warm air rises, it expands and cools triggering more condensation, the release of more latent heat, and a further increase in buoyancy, thus allowing more air to rise. A chain reaction (or feedback mechanism) is now in progress, as the rising temperatures in the center of the storm cause surface pressures to lower even more. Lower surface pressures encourage a more rapid inflow of air at the surface, more thunderstorms, more heat, lower surface pressure, stronger winds, and so on. If the storm is far enough from the equator (generally at least 8° of latitude), the Coriolis force will induce the converging winds into a counterclockwise circulation about the storm's area of lowest surface pressure. Meanwhile, air pressures near the top of the storm, in response to the latent heat warming, begin to rise. In response to higher pressures aloft, air begins to flow outward (diverge) around the top of the center of the cyclone. Analogous to a chimney, this upper-level area of high pressure vents the tropical system, preventing the air converging at the surface from piling up around the center. If this were to occur, surface pressures would rise inside the storm and ultimately weaken, or even destroy it.
Once sustained wind speeds reach 37 km (23 miles) per hour, the tropical disturbance is called a tropical depression. As winds increase to 63 km (39 miles) per hour, the cyclone is called a tropical storm and receives a name, a tradition started with the use of World War II vintage code names such as Able, Baker, Charlie, etc. For a number of years beginning in 1953, female names were used exclusively, and then the alternation of male and female names began in the late 1970s. Finally, when wind speeds reach 119 km (74 miles) per hour, the storm is classified as a hurricane. Even when the conditions are ripe for hurricane formation at the surface, the storm may not form if the atmospheric conditions aloft (5-10 km above the surface) are not favorable.

For example, around the area of 20° latitude, the air aloft is often sinking, due to the presence of the sub-tropical high—a semi-permanent high pressure system in the subtropical regions that facilitates sinking air motions (subsidence). The sinking air warms and creates a temperature inversion (an extremely stable air layer in which temperature increases with altitude, the inverse of the usual temperature profile in the lower atmosphere) known as the trade wind inversion. This warm layer is very stable, making it difficult for air currents to rise and form thunderstorms and (eventually) hurricanes. In addition, strong upper-level winds tend to decapitate developing thunderstorms by dispersing the latent heat and cutting off the storm’s source of fuel. At the surface, hurricanes can diminish rather quickly given the right conditions. These conditions include: (1) the storm moving over cooler water that can't supply warm, moist tropical air; (2) the storm moving over land, again cutting of the source of warm, moist air; and finally (3) moving into an area where the large-scale flow aloft is not favorable for continued development or sustainment. .

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