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What is a Tornado?
A tornado raises a cloud of dust as it moves along the ground.
Photograph courtesy NOAA

A tornado is a violently rotating column of air that extends from a thunderstorm to the ground.

It's often portended by a dark, greenish sky. Black storm clouds gather. Baseball-size hail might fall. A funnel suddenly appears, descending from a cloud.

The funnel hits the ground and roars forward with a sound like that of a freight train approaching. The tornado tears up everything in its path.

Some of the Earth's most violent events, nearly a thousand tornadoes—many of them deadly—touch down every year in the United States. Every U.S. state has experienced twisters, but Texas holds the record: an annual average of 120.

Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but most tornadoes occur in the United States.

Related to tornadoes, waterspouts are weak twisters that form over warm water. They sometimes move inland and become tornadoes.

Dust devils are small, rapidly rotating columns of air that are made visible by the dust and dirt they pick up. Dust devils are not associated with thunderstorms.

What Causes Tornadoes?
A twister touches down in Xenia, Ohio, on April 3, 1974.
Photograph courtesy NOAA

Tornadoes form when warm, humid air collides with cold, dry air.

The denser cold air is pushed over the warm air, usually producing thunderstorms. The warm air rises through the colder air, causing an updraft. This updraft will begin to rotate if winds vary sharply in speed or direction.

As the rotating updraft, called a mesocyclone, draws in more warm air from the moving thunderstorm, its rotation speed increases. Cool air fed by the jet stream, a strong band of wind in the atmosphere, provides even more energy.

Water droplets from the mesocyclone's moist air form a funnel cloud. The funnel continues to grow and eventually it descends from the cloud. When it touches the ground, it becomes a tornado.

The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation. About one in a thousand storms becomes a supercell, and one in five or six supercells spawns off a tornado.

Most tornadoes in the United States occur in Tornado Alley, a swath that stretches from Texas to Nebraska and also includes Colorado, Iowa, Indiana, Illinois, Kansas, Missouri, and Arkansas.

Tornado season begins in early spring for the states along the Gulf of Mexico. The season follows the jet stream—as it swings farther north, so does tornado activity. May generally has more tornadoes than any other month, but April's twisters are usually more violent.

Although they can occur at any time of the day or night, most tornadoes form in the late afternoon. By this time the sun has heated the ground and the atmosphere enough to produce thunderstorms.

Case Studies
Tri-State Tornado
Palm Sunday Outbreak
Super Outbreak
Oklahoma Outbreak
European Tornadoes
La Plata, Maryland

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