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The National Severe Storms Lab
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National Weather Service Tornado Preparedness Guide
The University of Illinois WW2010 project
The Tornado Project
What is a Tornado? by Charles Doswell
Texas Stormchaser Gene Moore's education page
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The National Severe Storms Laboratory estimates that more than 800 tornadoes occur each year in the United States. They have been recorded in every state, as well as in many other countries. In the United States, tornadoes are particularly common in the Great Plains, from the Rocky Mountains to the Appalachians, and they most often form in the spring and early summer.

Why and how tornadoes form is still not well understood. Often they form from rotating thunderstorms called mesocyclones. These form when winds in the bottom layer of the atmosphere just above the ground are moving in a different direction, and at a different speed, than winds in the layer directly above. Where the two layers touch each other, the conflicting winds "pull apart" or "shove around" the air at the boundary, until sometimes the air starts to roll over itself. This initial rotating is like that of a pencil if you roll it across a table with the palm of your hand. Sometimes, again for reasons not fully understood, this spinning movement is turned vertically -- as if you stood the spinning pencil up on its tip -- and if this type of rotation develops in a thunderstorm it is a mesocyclone. Tornadoes then form in several different ways, none of them well understood, from the rotating mesocyclone as the spinning air tightens and speeds up in small, localized regions.

One thing that is thought to tip the horizontal rotation vertically is the rising hot air beneath the thunderstorm. Such an updraft fuels the storm's power. Because of the updrafts, strong winds on the ground that are rushing toward a thunderstorm are one of the indicators that severe weather may be imminent. The sky is also usually very dark, the color of a bad bruise, and the clouds commonly have a sickly greenish cast.

Diagram of a Tornado
Diagram of a tornado. Click on the picture to see a detailed image.

Conditions that might spawn a tornado can be recognized long before things get that far, though, by anyone who studies the sky. Towering thunderheads are most likely to produce tornadoes. Heavy rain, hail, and lightning are usually associated with such storms. Tornadoes often form at the base of a wall cloud, which is a lowering of the underside of the thunderhead. Not all wall clouds produce tornadoes, but anyone who sees such signs -- especially large hail -- should pay close attention to local weather reports for storm watches and warnings

Thirty years ago, tornado warnings were issued when a spotter reported seeing a funnel touch the ground, and since rain and darkness often made it hard for spotters to see them, it was not uncommon for a tornado to suprise everyone in its initial path. Now Doppler radar is used to pinpoint tornado location and direction of travel with more reliability.