Hurricanes are intense rotating (cyclonic) storms that form over warm tropical waters. The names for tropical cyclones vary, depending on which part of the world they are formed. In the Eastern Pacific and Atlantic oceans, these storms are called hurricanes; in the Western Pacific, they are called typhoons; and in the Indian Ocean, they are called cyclones. Other local names also exist.
Hurricanes form in a series of stages, beginning as systems of strong thunderstorms. Under certain conditions, these storm systems will become tropical depressions and then strengthen into tropical storms. Tropical storms become hurricanes when winds exceed speeds of 74 mph (120 km/h).
Hurricanes remain strong only while over warm waters. Typically when hurricanes strike land and move inland, they immediately start to disintegrate. The radius of a major hurricane can be 100 mi (160 km) or greater. Major hurricanes can cause widespread destruction of property and loss of life, due primarily to storm surges—walls of seawater or the temporary rise in sea level associated with hurricanes—and to high winds along coastlines. Thunderstorms, hail, and tornados are also frequently produced by hurricanes, and major storms such as these may cause additional flooding and destruction hundreds of miles inland from the point of initial landfall.
Atlantic hurricanes usually move from east to west near the tropics, but when they migrate toward either the North or South Pole, moving into the mid-latitudes—the areas between the tropics and the polar regions—they can get caught up in the general west to east wind flows.
Hurricanes are currently measured on the Saffir-Simpson hurricane scale, which assigns hurricanes to categories—from Category 1 to Category 5—based upon their wind speed. A Category 1 storm has winds between 74 and 95 mph (120 and 153 km/h). Hurricanes in this category generally have a low storm surge and cause minimal damage to building structures. At the other extreme, a Category 5 hurricane has top sustained winds greater than 155 mph (250 km/h) and often comes ashore with a high storm surge with enormous destructive power. Although wind speed is an important indicator of strength, the storm surge is often the most damaging part of a hurricane.
With specific regard to hurricanes impacting the United States, the 2004 and 2005 hurricane seasons produced 4 of the 10 most intense Atlantic hurricanes on record. Hurricane Ivan slammed the U.S. Gulf Coast in 2004. In 2005, major hurricanes Katrina, Rita, and Wilma were among the most intense hurricanes recorded. As they crossed the warm waters of the Gulf of Mexico, all four of these storms reached Category 5 on the Saffir-Simpson scale.
In 2008, Hurricane Ike killed more than 600 people and caused massive flooding and homelessness in the Caribbean before crossing the Gulf to strike Galveston Island off the Texas coast near Houston. Although it struck the United States as a Category 2 storm, Ike was such a large storm—with a high storm surge more characteristic of a Category 4 or Category 5 storm—that it caused tremendous flooding and damage. Hurricane Ike became the third costliest hurricane to strike the United States behind Hurricanes Katrina and Andrew (1992).
Climate scientists and climate models vary in their predictions about the impact of global warming on hurricanes. Studies released late in 2009 suggested that global warming would result in fewer, but more intense, hurricanes. Although 2004 and 2005 produced a number of highly destructive hurricanes, there were also four years during the same decade (2000 to 2009) when no hurricane made landfall in the United States.
Atlantic hurricane formation is also strongly correlated to a number of climate and weather factors that influence hurricane formation and strength. For example, El Nińo events are historically associated with decreased hurricane activity in the Atlantic Ocean and Caribbean Sea.