What s the definition of a significant tornado?
A tornado is classified as “significant” if it does F2 or greater damage on the Fujita scale. (An F2 tornado has wind speeds above 111 mph.) Although only about 10% of U.S. tornadoes are classified as significant, they are responsible for the majority of deaths, with violent tornadoes (F4 or above, with winds of at least 166 mph) claiming 67% of the total U.S. tornado fatalities. This FAQ from the Storm Prediction Center has more about the basics of tornadoes, including info on the new enhanced Fujita scale, which will be implemented next February. This paper by Peggy Concannon has lots of good data on strong and violent tornadoes.
