TORNADOES -- Brian P. (Reseda, CA - about 25 miles from the Pacific Ocean) wonders a lot about tornadoes. For example, he wants to know
(a) what makes a tornado happen?
(b) can they happen by mountains?
(c) can a dust devil turn into a tornado? and
(d) is the United States the only place that has tornadoes?
He also wanted to know if tornadoes occur in California. -- 10/3/99
Tornadoes generally develop from thunderstorms. Not just any thunderstorm, but usually a storm that has very strong updrafts (rising air currents) and is bigger and long-lived than average. This isn't to say a tornado can't form from other types of thunderstorms, just that it is much rarer.
These types of large, long-lived storms are called "super-cells" (obviously a perfectly-fitting name). And they form most often in the central part of the United States. This is where three vastly different air masses often clash. Cold, dry air moves southward from Canada; warm, moist air moves northward from the Gulf of Mexico; and warm, dry air moves northeastward from the desert southwest and Mexico. Typically, when these air masses clash, a strong upper level jet stream is in the area. Put all of these together and the stage is set for major tornado outbreaks (such as the May 3, 1999 outbreak in Oklahoma and Kansas)
As it turns out, no other place on Earth has the geography that the central United States has. usually one or more air masses is missing from the situation. Because of this, the United States (especially the central United States) has the most tornadoes of any place. As before, tornadoes can, and have, occurred in other places. It is just that they are fewer in number and usually less severe. See the USGS map for information about tornado frequency.
Typically dust devils (relatively small swirls of air and dust) form on sunny days when the ground is heated strongly by the sun. They can also form when winds blow past an obstacle (like a building) at different speeds, creating temporary swirls. Dust devils won't become tornadoes.
Waterspouts, however, which may be very weak to tornadic in strength, can become (or be classified as) tornadoes when they come on land. This is relatively rare, but it can happen.
While tornadoes like to form and move over flat areas (central plains, for example), they have been reported in mountain areas and even in cities. perhaps the most significant mountain tornadoes occurred in Pennsylvania in 1985. One twister was reported as having an 80 mile or greater path as it moved up and down the ridges and mountains of the state during its over one hour lifetime.
For other information about tornadoes, including answers to other questions, photographs, graphs, and maps, check out the following web sites:
photo courtesy of NSSL
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