Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 349-A   September 13, 1969
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

A tornado is the smallest but most vicious and destructive of all storms. Tropical cyclones, called hurricanes in the Atlantic region and typhoons in the Western Pacific, are mild in comparison. A hurricane may be from 50 to 1000 miles across, travel thousands of miles during its life -- and cause enormous damage, but the fierce winds spiraling toward its center, or "eye", rarely exceed 150 miles per hour. No instrument has been able to measure the violently spinning winds near the center of a tornado cloud but they may reach velocities estimated to be in excess of 550 mph. It shrieks, howls and roars as its "foot" races across country, sometimes in hopscotch fashion, like an unpredictable evil demon.

A tornado is a whirling mass of air that is started -- not at the ground level as in the case of a "dust devil" -- but by powerful cross currents in the storm cloud level some thousands of feet above. Such turbulent uplifts occur most frequently in the spring months when cold masses of air from the north encounter warm masses from the south on sunny days when the air aloft is markedly colder than the earth-warmed air. The funnel-shaped cloud so formed may extend only slightly below the greenish black "mother cloud" and never touch the earth but, too often, the rotating air acts on the air below it and sets up a spinning column that extends down, down, down until it reaches the ground.

Sometimes the tapering column appears to dangle like an elephant's trunk; sometimes it hangs in a loose arch, like a great thick rope. Those whirling winds gyrate so violently -- always counterclockwise in the northern hemisphere -- that they greatly reduce the air pressure in the center. This abrupt reduction causes the air to expand and cool, forming the visible cloud. It is this vacuum-like center which sucks up roofs, causes the walls of buildings to burst outward and creates a boiling cloud of dust and debris. Objects as heavy as an anvil, an automobile or even a locomotive, may be carried away.

The annual number varies from 65 in 1919, to 262 in 1949. They have been reported during every month but are rare in fall and winter. May is the peak month, especially in the Great Plains and Midwestern states. Most tornadoes of early spring occur in the Gulf coastal states, and in autumn in the Atlantic coastal states are hardest hit. Except in the south, where the majority occur between midnight and noon, most tornadoes strike between 3 and 7 p.m.

The average width of a tornado's path is about 1200 feet, varying from 9 feet to 8 miles. It may touch the ground only briefly, or it may persist for several hours and its path of destruction be very wide. They usually move overland at a speed of about 40 miles per hour but sometimes one may stand stationary for several minutes, meanwhile whirling at tremendous speed, and then go on, or turn off in a new direction, or even make a wide looping swing. It may come from any direction but about seven-eighths of them come from the west -- usually the southwest. The terrible Tri-State Tornado of March 18,1925, traveled at 68 mph for 219 miles across Missouri, southern Illinois and Indiana, killing 606 people, and injuring 2000 others, in Illinois alone.

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