Tornadoes are violently spinning columns of air that extend from thunderclouds to the ground. These weather phenomena can produce some of the strongest winds on Earth and can cause incredible destruction. Although the exact mechanism by which a tornado develops is unknown, tornadoes often form in strong, rotating thunderstorms called supercell storms.
Tornadoes have been reported on every continent except Antarctica. They are most common in middle latitudes, where cold polar air meets warm tropical air. With more than one thousand tornadoes per year, the United States is the country with the most tornadoes. Florida has the most twisters per area, though most Florida tornadoes are fairly weak. There are two regions of the United States where tornadoes are more common: Tornado Alley, which stretches across the southern Great Plains; and Dixie Alley, which spans the southeastern states along the Gulf Coast.
Most tornadoes are relatively weak and short-lived, lasting less than 10 minutes. However, some can cause terrible destruction, often from flying debris. One of the most devastating tornadoes in U.S. history occurred in Joplin, Missouri on May 22, 2011. As the 1.6-kilometer (1-mile) wide twister traveled along the ground, it swept away homes and destroyed one-third of the buildings in the city. All in all, the storm caused nearly three billion dollars in damage, killed about 160 people, and injured more than 1,000 others
Enhanced Fujita Scale
The Fujita Scale was first developed in 1971 by Ted Fujita, a meteorologist at the University of Chicago in Illinois. Storm experts began using an enhanced version of the scale in 2007 that included more detailed descriptions of the damage. Unlike hurricanes, which are classified by measured wind speeds, tornado categories are based on wreckage after a tornado has struck, because the wind speeds are difficult to measure. After a tornado has passed, experts assess the damage, estimate wind speeds, and categorize tornadoes according to the Enhanced Fujita Scale, with ratings from EF-0 to EF-5.
EF-0
Wind gusts are estimated between 105 and 137 kilometers per hour (65 and 85 miles per hour). The environment sustained minor damage: tree branches are broken, some shallow-rooted trees are uprooted, and some chimneys are damaged.
EF-1
Wind gusts are estimated between 138 and 177 kilometers per hour (86 and 110 miles per hour). The environment sustained moderate damage: mobile homes are tipped over, windows are broken, roof tiles may be blown off, and some tree trunks have snapped.
EF-2
Wind gusts are estimated between 178 and 217 kilometers per hour (111 and 135 miles per hour). The environment sustained considerable damage: mobile homes are destroyed, roofs are damaged, debris flies in the air, and large trees are snapped or uprooted.
EF-3
Wind gusts are estimated between 218 and 266 kilometers per hour (136 and 165 miles per hour). The environment sustained severe damage: roofs and walls are ripped off buildings, small buildings are destroyed, and most trees are uprooted.
EF-4
Wind gusts are estimated between 267 and 322 kilometers per hour (166 and 200 miles per hour). The environment sustained devastating damage: well-built homes are destroyed, buildings are lifted off their foundations, cars are blown away, and large debris flies in the air.
EF-5
Winds of over 322 kilometers per hour (over 200 miles per hour) are present. The environment sustained incredible damage: well-built homes are lifted from their foundations, reinforced concrete buildings are damaged, the bark is stripped from trees, and car-sized debris flies through the air.
