Just as earthquakes are rated on the Richter scale, tornadoes are rated on a scale from F-0 to F-5. F-0 and F-1 storms can collapse chimneys and toss mobile homes off their foundations. They have the strength to push cars off roads. F-2 and F-3 tornadoes can tear away roofs and lift and toss heavy cars. F-4 and F-5 tornadoes are so strong they can pick up and carry entire buildings a distance, toss steel beams, and crunch cars.

F-4 and F-5 tornadoes are rarer, but are responsible for 2/3 of the 120 or so tornado deaths in the United States every year.

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Forces of Nature: ThinkQuest 2000 (Team #C003603)

http://library.thinkquest.org/C003603/english/tornadoes/tornadostrength.shtml