Tornadoes — sometimes called twisters — are rotating columns of air that connect a cumulonimbus cloud with the surface of the Earth. One of the most violent of all weather phenomena, tornadoes are very dangerous and can cause massive damage.

Tornadoes come in all sorts of sizes and shapes. Most often, they take the form of a funnel-shaped cloud with a narrow end touching the Earth. They are usually surrounded by large clouds of debris and dust.

Most tornadoes feature wind speeds under 110 miles per hour, span about 250 feet in width and only travel a few miles before dissipating. The largest, most destructive tornadoes, though, can boast wind speeds of more than 300 miles per hour, span more than two miles in width and remain on the ground for dozens of miles.

Weather experts use the Fujita scale to rate tornadoes based on the damage they cause. Tornadoes can receive a Fujita scale rating from F0 to F5.

For example, an F0 tornado — the weakest possible — can damage trees but not sturdy structures. On the opposite end of the scale, an F5 tornado — the strongest possible — can tear buildings off their foundations and even deform huge skyscrapers.

"Tornado Alley" is a term that the media and weather experts use to refer to the area of the United States where the strongest tornadoes occur most frequently. There is no official definition of Tornado Alley, but the area between the Rocky Mountains and the Appalachian Mountains is commonly considered to define the area.

The term was first used by Jennifer Wiley in 1904. The heart of Tornado Alley consists of the Texas Panhandle, Oklahoma, Kansas, Nebraska, eastern South Dakota and the Colorado Eastern Plains. Although no state avoids tornadoes entirely, the strongest ones tend to happen in these areas.

Texas reports the most tornadoes annually, but Kansas and Oklahoma report more tornadoes per land area than even Texas. Florida also experiences a high number of tornadoes each year, although they tend to be weaker storms than those in Tornado Alley.

Weather experts believe that strong tornadoes occur so frequently in Tornado Alley because cold, dry air from Canada and the Rocky Mountains meets warm, moist air from the Gulf of Mexico and hot, dry air from the Sonoran Desert, which is located in the southwestern part of the United States. When these air masses combine with atmospheric instability, they produce intense thunderstorms that spawn destructive tornadoes.

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