|MadSci Network: Earth Sciences|
The occurrence of tornadoes is not restricted to the central plains of the United States. Every state in the U.S. has reported tornadoes. Tornadoes also occur in Australia, Korea, Europe, Canada and Russia. However, the vast majority of them occur in the area roughy bounded by the Rocky Mountains in the west, Texas in the south, North to the Dakotas, and east to the Appalachian Mountains; the so-called Tornado Alley.
Conditions in this general area are especially favorable for large numbers of tornadoes. These conditions include:
Our written weather records for tornadoes don’t really start in the central plains until the late 1800’s when more people began to settle. I have included the oldest known picture of a tornado, taken on August 28, 1884 near Howard, South Dakota (below).
Tornadoes require a special set of atmospheric conditions to occur regularly. The central plains of the U.S. have these conditions, and probably have had them as long as the North American continent has been relatively close to its present position, and as long as the climate has been similar to now. My best estimate is the the central plains have been a ‘Tornado Alley’ since the last ice age, approximately 10,000 years ago. ( http://www.nature.ca/notebooks/english/iceage.htm)
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