III.

Classification

Direct measurements of tornado wind speeds are difficult (and dangerous) to obtain. In 1971 Theodore Fujita, a meteorology professor at the University of Chicago, devised a classification system linking the degree of damage to humanmade structures to possible wind speeds. In 1973 this system, known as the Fujita or F-scale system, was adopted as the official tornado classification system of the National Weather Service (NWS). Using the F-scale system, the NWS ranks tornado damage as light to moderate (F0 and Fl), considerable to severe (F2 and F3), or devastating to incredible (F4 and F5). The weakest tornadoes (F0) may damage chimneys and signs, whereas the most violent tornadoes (F5) can blow houses completely off their foundations. Although media reports may sometimes refer to a tornado as an F6, this ranking is not recognized by the NWS.

However, the F-scale system applies only in regions where humanmade structures exist. Also, scientists are able to correlate F-scale values only roughly with wind speeds. For instance, a wind speed of 145 km/h (90 mph) might do minor F0 damage to a well-constructed building but significant F2 damage to a poorly constructed building. Scientists estimate that F0 tornadoes may have wind speeds up to 110 km/h (70 mph), while F5 tornadoes may have wind speeds somewhere in the range of 420 to 480 km/h (260 to 300 mph). While the F-scale system remains a convenient means for scientists to classify and discuss the intensity of tornadoes, the limitations of the scale have inspired attempts to design a more sophisticated modeling system.

In the United States, 75 percent of the tornadoes rate F0 or F1 in strength. Most remaining tornadoes rate F2 or F3, with only 1 percent rating F4 or F5. Usually no more than one or two tornadoes per year reach F5 strength. On the other hand, the few F4 and F5 tornadoes account for 67 percent of the fatalities caused by tornadoes.