What was the Fujita scale based on and why?

The Fujita ("F") scale is based on the severity of the damage which is used to provide a basic wind speed estimate. It uses this technique because when the was created in 1971 we had no means of obtaining direct wind measurements from inside a tornado. More recently in 2007 the wind speed estimates have been adjusted to more accurately correspond with the damage levels. This is called the Enhanced Fujita ("EF") scale. Although it is now possible to get direct wind measurements from a tornado, such measurements are rare and difficult to obtain so that only a handful of tornadoes have such measurements taken. So damage must still be used as that primary means of rating tornadoes.