The Enhanced Fujita Scale: How Tornadoes are Rated

The original Fujita scale is named after Dr. Ted Fujita, a University of Chicago severe storms research scientist who came up with the scale in 1971. Dr. Fujita's scale, which ranges from F0 to F5, is based upon the type and severity of damage the tornado produced. At that time, there were very few actual measurements of tornado wind speeds that he could relate to the damage, but he used them -- together with a lot of insight -- to devise approximate wind speed ranges for each damage category.

In subsequent years, structural engineers and meteorologists have examined damage from many tornadoes. They use knowledge of the wind forces needed to damage or destroy various buildings and their components to estimate the resultant wind speeds. The engineers and scientists eventually determined that the original F-Scale wind speeds were too high for categories F3 and higher.

http://www.weather.com/storms/tornado/news/enhanced-fujita-scale-20130206