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Tornadoes are defined as an aggressively rotating column of air which is in contact with both a cloud base and the earth. Tornadoes typically have a visible funnel, with the narrow end touching the surface of the earth.

Most tornadoes have winds of 100mph or less, are approximately 90 meters wide, and travel a few miles before fizzling out. Some tornadoes can have winds of more than 280mph, can be more than a mile across, and stay on the ground for tens of miles.

Tornadoes have been observed on every continent on earth except Antarctica, the majority the world's tornadoes occur in the United States and in particular Tornado Alley. Other areas which commonly experience tornadoes around the world include New Zealand, Australia, Canada, central Europe, south-central and eastern Asia, South America, and Southern Africa.

The life cycle of a tornado begins when a strong thunderstorm develops a rotating mesocyclone high up in the atmosphere. As rainfall in the storm increases, it drags with it an area of quickly descending air known as the rear flank downdraft. This downdraft accelerates as it approaches the ground, and drags the rotating mesocyclone towards the ground with it.

As the mesocyclone nears the ground, a visible funnel appears from the base of the storm clouds. As the funnel drops, the downdraft reaches the ground, creating a gust of wind that can cause damage far from the tornado.

At the start of the cycle, the tornado has a good source of warm, moist inflow to power it, and it grows. During its mature stage, lasting anywhere from minutes to more than an hour, a tornado often causes the most damage. Meanwhile, the RFD, now an area of cool surface winds begins to wrap around the tornado and chokes off the air supply, the tornado begins to deteriorate.

As the tornado enters the dissipating stage, its associated mesocyclone often weakens as well, as the rear flank downdraft cuts off the air flow powering it. As the first mesocyclone and associated tornado dissipate, the storm's inflow is concentrated into a new area closer to the center of the storm. If a new mesocyclone develops, the cycle may begin again, generating a new tornado, and in some cases there may be 2 or more funnels touching the earth at any one time.

Find out more about Tornado Alley in the United States.