This scale was developed in 1971 by Dr. T. Theodore Fujita of the University of Chicago and Allen Pearson, then Director of the National Severe Storms Forecast Center (NSSL).
The scale they devised, based on six categories, converts the degree and type of damage caused by a tornado into an estimation of the wind speeds inside the funnel.
The classification of a tornado must, therefore, be done after the event, based on the degree of damage.
As of February 1, 2007, the Fujita Scale has been replaced by the Enhanced Fujita Scale (EF Scale), now used operationally to estimate tornado intensity and wind speed.
Assigning and Enhanced Fujita Scale rating involves a much more detailed inspection of damage than with the original F-Scale, with wind speeds now estimated based upon guidelines for 28 different types of structures and objects. An EF rating is then applied to the tornado based upon the damage including the highest wind speed.
EF0 65-85 mph
EF1 88-110 mph
EF2 111-135 mph
EF3 136-165 mph
EF4 166-200 mph
EF5 over 200 mph