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Tornadoes - How Tornadoes Form


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What Is a Tornado?
Virginia Tornado 4/29/08

Local residents check out the damage of vehicles at a mall after it was damaged by a tornado April 29, 2008 in the King's Fork area of Suffolk, Virginia. Three Tornadoes touched down in central and southeastern Virginia injuring at least 200 people.

Photo by Alex Wong/Getty Images

A tornado is a violent column of rotating air made visible as they pick up debris on the ground or in the air. A tornado is usually visible, but not always. The important aspect of the definition is that the tornado or funnel cloud is in contact with the ground. The funnel clouds appear to extend downward from cumulonimbus clouds. A point to keep in mind is that this definition is not a truly accepted definition. According to Charles A. Doswell III of the Cooperative Institute for Mesoscale Meteorological Studies, there is actually no real definition of a tornado that has been universally accepted and peer-reviewed by the scientific community.

One idea that is generally accepted is that tornadoes are one of the worst, and most violent, of all the types of severe weather. Tornadoes can be considered billion-dollar storms if the storm lasts sufficiently long enough, and has enough wind speed to do maximum property damage. Fortunately, most tornadoes are short-lived, lasting for only about 5-7 minutes on average.

Tornado Rotation

Most tornadoes in the Northern Hemisphere rotate counter-clockwise or cyclonically. Only about 5% of tornadoes in the Northern Hemisphere rotate clockwise or anticyclonically. While at first it would seem this is a consequence of the Coriolis effect, tornadoes are over almost as quickly as they start. Therefore, the influence of the Coriolis effect on rotation is negligible.

So why do tornadoes tend to rotate counter-clockwise? The answer is that the storm moves in the same general direction as the low pressure systems that spawn them. Since low pressure systems rotate counterclockwise (and this is due to the Coriolis effect), tornado rotation also tends to be inherited from the low pressure systems. As winds get pushed upwards in the updraft, the prevailing direction of rotation is counterclockwise.

Tornado Locations
Each year, hundreds of tornadoes affect areas all around the world. Yet the largest number of tornadoes occur in the Midwest United States in an area known as tornado alley. In the United States, a unique combination of factors including local geology, proximity to water, and movement of frontal systems make the United States a prime location for the formation of tornadoes. In fact, there are 5 key reasons the US is the hardest hit with tornadoes.

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