The Science of Tornadoes

At least 17 people are dead after a string of 31 tornadoes hit states in the Southern and Midwestern U.S. on Sunday night.

A tornado, or twister, is a violently rotating column of air that extends between the Earth's surface and a cloud, usually a cumulonimbus cloud. Most tornadoes last for less than ten minutes, says Harold Brooks, a research meteorologist with the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory (NSSL) in Norman, Oklahoma. Large tornadoes usually last longer—around 30 minutes, Brooks adds. The most powerful twisters have wind speeds of more than 300 miles (483 kilometers) per hour, which can rip buildings off their foundations. They can be more than two miles (3.2 kilometers) wide, and can spin across the ground for dozens of miles. The more common tornadoes have wind speeds of less than 110 miles (177 kilometers) per hour, are about 250 feet (76 meters) across, and travel only a few miles before they dissipate. Tornadoes kill an average of 60 people a year in the U.S., mostly from flying or falling debris, reports NOAA. (See "Interactive: Forces of Nature.") Half of those deaths are caused by the strongest one percent of the most violent storms, says Brooks. continue