| MadSci Network: Earth Sciences |
Tornadoes are more frequent in the United States and Canada than anywhere else in the world. The central U.S. has even been called "Tornado Alley". 75% of all tornadoes occur between March and July.
Almost all tornadoes form from thunderstorms at the boundary between air masses (like a cold front) where cold air pushing over warm air causes the warm air to rise upward and the cold air to sink down. The clash of warm and cold air causes condensation, rain, hail, lightning, and strong winds. Tornadoes are often associated with hail and hard rain. Fast vertical air movement is typical in thunderstorms, and this is what contributes to the formation of tornadoes.
Tornadoes are especially associated with a type of thunderstorm called a supercell. This is a large, very strong thunderstorm with "cyclonic" rotating winds. A "wall cloud" sometimes can be seen hanging down from the cloud base at the edge of the storm. The wind flows into and around the base of the storm, then upward inside it. Rising columns of air tend to rotate and develop "vorticity". The air circulation in the storms lower layers tightens into narrow cylinders of rotation called "vortex tubes". As a cylinder gets tighter the wind speeds up and the cylinder stretches toward the ground. If the vortex reaches the ground it is called a tornado.
A funnel-shaped cloud is often seen in a tornado because the very low pressure inside the vortex causes water vapor to condense. However, the tornado is actually the wind, not the cloud, so a tornado may cause damage even if the funnel cloud does not reach the ground.
Here are some links to tornado sites you should check out:
http://cc.usu.edu/~kforsyth/Tornado.html
You can see a good illustration of a supercell at:
http://www.sirs.com/partner/snow/snow595/snow595.htm
I hope this helps answer your question.
---- Sean Sherlock