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Hurricane (飓风) Hurricane is a name given to violent storms that originate over the tropical (热带的) or subtropical waters of the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or North Pacific Ocean east of the International Date Line. Such storms over the North Pacific west of the International Date Line are called typhoons (台风); those elsewhere are known as tropical cyclones (热带气旋), which is the general name for all such storms including hurricanes and typhoons. These storms can cause great damage to property and loss of human life due to high winds, flooding, and large waves crashing against shorelines. The deadliest natural disaster in the United States history was caused by a hurricane that struck the coast of Texas in 1990. The costliest natural disaster in U.S. history stemmed from Hurricane Katrina in 2005. How hurricanes form Oceans can become warm enough in the summer for hurricanes to develop, and the oceans also retain summer heat through the fall. As a result, the hurricane season in the Atlantic Basin, which comprises the Atlantic Ocean, Caribbean Sea, and the Gulf of Mexico, runs from June 1 through November 30. At least 25 out-of-season storms, however, have occurred from 1887 through 2003, and 9 of these strengthened into hurricanes for at least a few hours. Hurricanes weaken and die out when cut off from warm, humid air as they move over cooler water or land but can remain dangerous as they weaken. Hurricanes and other tropical cyclones begin as disorganized clusters of showers and thunderstorms. When one of these clusters becomes organized with its winds making a complete circle around a center, it is called a tropical depression (热带低气压). When a depression’’s sustained winds reach 63 km/h or more, it becomes a tropical storm and is given a name. By definition, a tropical storm becomes a hurricane when winds reach 119 km/h or more. For a tropical depression to grow into a hurricane, winds from just above the surface of the ocean to more than 12,000 m in altitude must be blowing from roughly the same direction and at the same speed. Winds that blow in opposite directions create wind shears-different wind speeds or direction at upper and lower altitudes (海拔)—that can prevent a storm from, growing. Characteristics of hurricanes A hurricane consists of bands of thunderstorms that spiral (盘旋) toward the low-pressure center, or "eye" of the storm. Winds also spiral in toward the center, speeding up as they approach the eye. Large thunderstorms create an "eye wall" around the center where winds are the strongest. Winds in the eye itself are nearly calm, and the sky is often clear. Air pressures in the eye at the surface range from around 982 hectopascals (百帕) in a weak hurricane to lower than 914 hectopascals in the strongest storms. In a large, strong storm, hurricane-force winds may be felt over an area with a diameter of more than 100 km. The diameter of the area affected by gale winds and torrential rain can extend another 260 km or more outward from the eye of the storm. The diameter of the eye may be less than 16 km in a strong hurricane to more than 48 km in a weak storm. The smaller the diameter of the eye, the stronger the hurricane winds will be. A hurricane’’s strength is rated from Category 1, which has winds of at least 119 km/h, to Category 5, which has winds of .more than 249 km/h. These categories, known as the Saffir-Simpson hurricane scale, were developed in the 1970s. In the tropics, hurricanes move generally east to west, steered by global-scale winds. Hurricane, typhoons, and cyclones usually "recurve" in the direction of either the South Pole in the Southern Hemisphere or the North Pole in the Northern Hemisphere. Eventually the storms move toward the east in the middle latitudes, but not all storms recurve. Hurricanes travel at varying rates. In the lower latitudes the rate usually ranges from 8 to 32 km/h, and in the higher latitudes it may increase to as much as 80 km/h. In addition to generating large weaves that travel out in all directions, hurricane winds pile up water. This piling up of water is known as a storm surge, and it can raise the sea level more than 6 m when the storm hits land. The deadliest natural disaster in U.S. history was the 1990 Galveston Texas hurricane, which killed an estimated 8,000 people. The storm surge accounted for most of the deaths. The costliest natural disaster in U.S. history was caused by the storm surge, created by Hurricane Katrina in 2005. The hurricane’’s storm surge burst levees protecting New Orleans, Louisiana, flooding the city and forcing a complete evacuation. The worst tropical storm disaster since the 20th century began was a 1970 cyclone that struck East Pakistan when a storm surge killed an estimated 300,000 people. Since the last third of the 20th century, floods and landslides from heavy rain were the leading cause of hurricane and tropical storm deaths. In October 1998 Hurricane Mitch’’s torrential rain caused floods and landslides that killed more than 9,000 people with another 9,000 missing and presumed dead in Central America, according to the U.S. National Hurricane Center. Although the hurricane death toll steadily declined in the United States during the 20th century and at the start of the 21st century, the costs of damage soared as coastal populations grew and the value of property exceeded population, growth. Before Hurricane Katrina, the costliest U.S. natural disaster was Hurricane Andrew, which hit the Miami, Florida, metropolitan area in 1992, causing $26. 5 billion in damages, including both insured and uninsured losses. Some estimates of Hurricane Katrina’’s damages ran as high as $125 billion. In addition, federal relief efforts were expected to cost in the hundreds of billions. Hurricanes and global warming In recent years concerns have arisen that a general warming of the Earth’’s climate could increase the numbers or strength of hurricanes and tropical cyclones. In a January 2001 report the United Nations Intergovernmental Panel on Climate Change said it had found no evidence that peak wind speeds or amounts of precipitation (降水量) in tropical cyclones had increased in the last half of the 20th century. Long-term records do not provide enough information to conclude whether the global total of tropical cyclones increased during the 20th century. But detailed records of Atlantic, Caribbean, and Gulf of Mexico hurricanes show that the numbers of storms increase and decrease in cycles. Many hurricane researchers think the cycles are related to changes in Atlantic Ocean temperatures that last decades. From the late 19th century through the 1980s about one-third of the major hurricanes that formed in the Atlantic hit the United States, which means around ten such hurricanes could have been expected to hit from 1995 through 2003. Yet for reasons atmospheric scientists do not understand, only three such hurricanes hit the United States from 1995 through 2003. Researchers who study hurricanes and climate say that the computer models used to predict global climate changes do not look at weather in the detail needed to forecast whether a warmer world would increase the number or strength of hurricanes. On the other hand, scientists have no reason to expect fewer or weaker hurricanes to form than has occurred in the past. They also have no reason to think that many storms will miss the United States as they did in the 1990s and early 2000s. This means that no matter how global climate change affects hurricanes, increased population along the coasts places more people and property in harm’’s way. The winds are the strongest in the eye of a hurricane.