Photograph courtesy NASA/GSFC
Hurricanes are giant, spiraling tropical storms that can pack wind speeds of over 160 miles (257 kilometers) an hour and unleash more than 2.4 trillion gallons (9 trillion liters) of rain a day. These same tropical storms are known as cyclones in the northern Indian Ocean and Bay of Bengal, and as typhoons in the western Pacific Ocean.
The Atlantic Ocean’s hurricane season peaks from mid-August to late October and averages five to six hurricanes per year.
Hurricanes begin as tropical disturbances in warm ocean waters with surface temperatures of at least 80 degrees Fahrenheit (26.5 degrees Celsius). These low pressure systems are fed by energy from the warm seas. If a storm achieves wind speeds of 38 miles (61 kilometers) an hour, it becomes known as a tropical depression. A tropical depression becomes a tropical storm, and is given a name, when its sustained wind speeds top 39 miles (63 kilometers) an hour. When a storm’s sustained wind speeds reach 74 miles (119 kilometers) an hour it becomes a hurricane and earns a category rating of 1 to 5 on the Saffir-Simpson scale.
Hurricanes are enormous heat engines that generate energy on a staggering scale. They draw heat from warm, moist ocean air and release it through condensation of water vapor in thunderstorms.
Hurricanes spin around a low-pressure center known as the “eye.” Sinking air makes this 20- to 30-mile-wide (32- to 48-kilometer-wide) area notoriously calm. But the eye is surrounded by a circular “eye wall” that hosts the storm’s strongest winds and rain.
These storms bring destruction ashore in many different ways. When a hurricane makes landfall it often produces a devastating storm surge that can reach 20 feet (6 meters) high and extend nearly 100 miles (161 kilometers). Ninety percent of all hurricane deaths result from storm surges.
A hurricane’s high winds are also destructive and may spawn tornadoes. Torrential rains cause further damage by spawning floods and landslides, which may occur many miles inland.
The best defense against a hurricane is an accurate forecast that gives people time to get out of its way. The National Hurricane Center issues hurricane watches for storms that may endanger communities, and hurricane warnings for storms that will make landfall within 24 hours.
More About Hurricanes
Despite unexpectedly low damage assessments, the hurricane was bigger and longer lasting than it had any right to be, experts say.
In the wake of Irene, New York, Virginia Beach, and other U.S. East Coast cities recover amid flooding and the debris of damaged homes.
While hurricanes, droughts, floods, and storm surges are natural events, the degree of disaster is often now heavily influenced by humans.
Houses wiped off the map, submerged islands, and flooded cemeteries—see how sites hit by Hurricane Katrina five years ago are faring in 2010.
New Orleans plans to pipe semi-treated sewage into a bayou to help regrow a cypress-tupelo wetland and protect the Lower Ninth Ward.
Flaming, oily hurricanes and "black rain" are no danger to Gulf residents on Katrina's fifth anniversary—or to anyone, anywhere, experts say.
When seas change color, storms change direction, a new study suggests.
@NatGeoGreen on TwitterTweets by @NatGeoGreen
The Great Energy Challenge
An initiative to help you understand our current energy situation.
See how you measure up against others, and how changes at home could do tons to protect the planet.
How much does it take to keep you cool in summer?
Special Ad Section
The World's Water
NG's new Change the Course campaign launches. When individuals pledge to use less water in their own lives, our partners carry out restoration work in the Colorado River Basin.
A special series on how grabbing water from poor people and future generations threatens global food security, environmental sustainability, and local cultures.