Harold Brooks, head of the Mesoscale Applications Group at the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Okla., explains.
The central part of the U.S. gets many tornadoes, particularly strong and violent ones, because of the unique geography of North America. The combination of the Gulf of Mexico to the south and the Rocky Mountains to the west provides ideal environmental conditions for the development of tornadoes more often there than any other place on earth.
To understand this phenomenon, consider the basic ingredients of a thunderstorm: warm, moist air near the ground; dry air aloft (between altitudes of about three and 10 kilometers; and some mechanism such as a boundary between two air masses to lift the warm, moist air upwards. Storms that produce strong tornadoes are also most likely to occur when the horizontal winds in the environment increase in speed and change with increasing altitude. In the most common directional change of this kind, the surface winds blow from the equatorward direction at the surface and out of the west a few kilometers above the ground. When this wind pattern occurs in the central part of the U.S., the surface winds come from the direction of the Gulf of Mexico, bringing in warm, moist air at the surface, and the winds aloft come from over the Rocky Mountains and are relatively dry. (Lifting air and heating it over a wide, high range of mountains is an ideal way to dry it.) As a result, when the winds over the central part of the U.S are correct for making thunderstorms, they often bring together the right combination of the vertical temperature and moisture profile most likely to produce tornadoes.
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No other part of the world has the combination of a warm, moist air source on the equatorward side and a wide, high range of mountains to the west that extends for thousands of kilometers from north to south that provides the right atmospheric conditions for frequent tornadoes. The Andes Mountains are not as wide as the Rockies, and the Himalayas don¿t extend as far north and south. Typically, air coming onshore off the Gulf has spent a longer time over the warm water than air coming onshore off of the Mediterranean Sea and is moister as a result. The Drakensberg Plateau in South Africa is not as high as the Rockies. In summary, other regions of the world that occasionally get strong tornadoes don¿t experience the combination of all the necessary tornado elements as often as the central U.S. does.
Answer originally published August 11, 2003.