Tornado (1) A rotating column of air usually accompanied by a funnel-shaped downward extension of a Cumulonimbus Cloud and having a Vortex several hundred yards in diameter whirling destructively at speeds of up to 500 miles per hour (800 kilometers per hour). (2) A violent thunderstorm in western Africa or nearby Atlantic waters. (3) A whirlwind or hurricane. Also referred to as a twister. Scientists rank a tornado’s intensity and estimate its wind speed based on observed damage, using a scale developed by the late University of chicago researcher Tetsuya Theodore Fujita. The rankings are as follows: 1 FO: Gale Tornado – Wind speed: 40-72 miles per hour. Damage: Some damage to chimneys, breaks branches off trees, pushes over shallow-rooted trees, damages signs. 2 F1: Moderate Tornado – Wind speed: 73-112 miles per hour. Damage: Peels roof coverings, mobile homes pushed off foundations or overturned, moving autos pushed off the roads, attached garages may be destroyed. 3 F2: Significant Tornado – Wind speed: 113-157 miles per hour. Damage: Considerable damage. Roofs torn off frame houses, mobile homes demolished, boxcars pushed over, large trees snapped or uprooted, light objects become missiles. 4 F3: Severe Tornado – Wind speed: 158-206 miles per hour. Damage: Roofs and some walls torn off well-constructed houses, trains overturned, most trees in forests uprooted. 5 F4: Devastating Tornado – Wind speed: 207-260 miles per hour. Damage: Well-constructed houses leveled, structures with weak foundations blown off some distance, cars thrown, large missiles generated. 6 F5: Incredible Tornado – Wind speed: 261-318 miles per hour. Damage: Strong frame houses lifted off foundations and carried considerable distances, automobile-size missiles fly thorugh the air for more than 100 meters, trees debarked, steel-reinforced concrete structures badly damaged. 7 F6 – Although he called it “inconceivable”, Fujita left open the possibility a tornado could cause a small area of damage even worse than F5 intensity. But he said evidence of an F6 tornado, with wind speeds up to 379 miles per hour, would probably be masked by damage from surrounding F4 and F5 winds. On average, only one F5 tornado hits the United States each year. F5 twisters struck in 1999 in Norman, Oklahoma, in 1998 in Waynesboro, Tennessee and Pleasant Grove, Alabama, in 1997 in Jarrell, Texas, in 1996 in Oakfield, Wisconsin, in 1992 in Chandlier, Minnesota, and in 1990 in Plainfield, Illinois, Goessel, Kansas, and Hesston, Kansas. In one of the worst U.S. tornadoes on record, seven F5s struck the Midwest on April 3, 1974, including at Xenia, Ohio.