• The vast majority of hurricane deaths and damage are caused by relatively infrequent, yet powerful storms. The costliest natural disaster in U.S. history, with damages in excess of $25 billion, was Hurricane Andrew in 1992.

• Most hurricanes form between the latitudes of 5° and 20° over all tropical oceans except the South Atlantic and eastern South Pacific. The North Pacific has the greatest number of storms, averaging 20 per year. In the western Pacific, hurricanes are called typhoons, and in the Indian Ocean, they are referred to as cyclones.

• A steep pressure gradient generates the rapid, inward spiraling winds of a hurricane. As the warm, moist air approaches the core of the storm, it turns upward and ascends in a ring of cumulonimbus towers and forms a doughnut-shaped wall called the eye wall. At the very center of the storm, called the eye, the air gradually descends, precipitation ceases, and winds subside.

• A hurricane is a heat engine fueled by the latent heat liberated when huge quantities of water vapor condense. They develop most often in late summer when ocean waters have reached temperatures of 27°C (80°F) or higher and are thus able to provide the necessary heat and moisture to the air. The initial stage of a tropical storm's life cycle, called a tropical disturbance, is a disorganized array of clouds that exhibits a weak pressure gradient and little or no rotation. Tropical disturbances that produce many of the strongest hurricanes that enter the western North Atlantic and threaten North America often begin as large undulations or ripples in the trade winds known as easterly waves.

  

• Each year, only a few tropical disturbances develop into full-fledged hurricanes that require minimum wind speeds of 119 kilometers per hour. When a cyclone's strongest winds do not exceed 61 kilometers per hour, it is called a tropical depression. When winds are between 61 and 119 kilometers per hour, the cyclone is termed a tropical storm. Hurricanes diminish in intensity whenever they (1) move over ocean waters that cannot supply warm, moist tropical air, (2) move onto land, or (3) reach a location where large-scale flow aloft is unfavorable.

• Although damage caused by a hurricane depends on several factors, including the size and population density of the area affected and the near-shore bottom configuration, the most significant factor is the strength of the storm itself. The Saffir-Simpson scale ranks the relative intensities of hurricanes. A 5 on the scale represents the worst storm possible, and a 1 is the least severe. Damage caused by hurricanes can be divided into three classes: (1) storm surge, which is most intense on the right side of the eye where winds are blowing toward the shore, occurs when a dome of water 65 to 80 kilometers (40 to 50 miles) wide sweeps across the coast near the point where the eye makes landfall, (2) wind damage, and (3) inland freshwater flooding, which is caused by torrential rains that accompany most hurricanes.

  

• North Atlantic hurricanes develop in the trade winds, which generally move these storms from east to west. Today, because of early warning systems that help detect and track hurricanes, the number of deaths associated with these violent storms have been greatly reduced. Because the tropical and subtropical regions that spawn hurricanes consist of enormous areas of open oceans, meteorological data from these vast regions are provided primarily by satellites. Other important sources of hurricane information are aircraft reconnaissance, radar, and remote, floating instruments called data buoys. Using data from the observational tools, meteorologists can issue an announcement, called a hurricane watch, aimed at specific coastal areas threatened by a hurricane, generally within 36 hours. By contrast, a hurricane warning is issued when sustained winds of 119 kilometers per hour or higher are expected within a specified coastal area in 24 hours or less. Two important factors in the watch-and-warning decision process are (1) adequate lead time and (2) attempting to keep overwarning at a minimum.

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