What is the F-scale?
Dr. T. Theodore Fujita developed a damage scale for winds, including tornadoes, that is supposed to relate the degree of damage to the intensity of the wind. This scale was the result. The F-scale should be used with great caution. Tornado wind speeds are still largely unknown; and the wind speeds on the F-scale have never been scientifically tested and proven. Different winds may be needed to cause the same damage depending on how well built a structure is, wind direction, wind duration, battering by flying debris, and many other factors. Also, the process of rating the damage itself is largely a judgment call—quite inconsistent and arbitrary. Even meteorologists and engineers highly experienced in damage survey techniques may come up with different F-scale ratings for the same damage. Even with all its flaws, the F-scale is the only widely used tornado rating method, and probably will remain so until ground-level winds can be measured in most tornadoes.
