The Fujita Scale

F0 40-72 mph Gale Tornado--light damage. Some damage to chimneys; break branches off trees; push over shallow-rooted trees; damage sign boards.
F1 73-112 mph Moderate Tornado--moderate damage. The lower limit is the beginning of hurricane wind speed; peel surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads.
F2 113-157 mph Significant Tornado--considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated.
F3 158-206 mph Severe Tornado--severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
F4 207-260 mph Devastating Tornado--devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.
F5 261-318 mph  Incredible Tornado--Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile-sized missiles fly through the air in excess of 100 meters (109 yds.); trees debarked; incredible phenomena will occur.
F6 - F12 319 mph - Up The maximum wind speeds of tornadoes are not expected to reach the F6 wind speeds.

    Tornadoes in the U.S. are classified according to a system known and developed by Professor Theodore Fujita, retired professor at the University of Chicago, and Dr. Alan Pearson, former director of the National Severe Storm Forecast Center (NSSFC), who devised the system in 1971. This scale relates the wind speed of a tornado to the amount of damage done. This scale is not a perfect system for linking damage to wind speed, but it has advantages over what had been used previously.

    An important thing to remember is that the size of a tornado is not necessarily an indication of its intensity. Large tornadoes can be weak and small tornadoes can be strong. The Fujita Scale is based on damage, not the appearance of the funnel. Storm spotters, storm chasers and other weather observers often try to estimate the intensity of a tornado when they are in the field, basing their judgment on the rotational speed and amount of debris being generated as well as the width. However, the official estimate is made after the tornado has passed. Personnel from the National Weather Service office that issued the warning survey the site to determine the F-Scale rating. Sometimes they call in experts from out of the area. Aerial surveys are occasionally done after violent tornadoes to determine the exact damage track.

    Approximately 1000 tornadoes occur each year in the United States. Records indicate that the vast majority of tornadoes are either weak or do damage that can only be attributed to a weak tornado. Only a small percentage (less than 1%) of tornadoes can be classed as violent. 76% are classified as weak and 24% are classified as strong. It is quite possible that an even higher percentage of all tornadoes are weak. There is evidence that 1000 or more additional weak tornadoes may occur each year and go completely undocumented.

Fujita Scale was simple enough to use in daily practice without involving much additional expenditure of time or money. From a practical point of view, it is doubtful that any other system would have found its way into widespread accepted use, even to this day. The entire premise of estimating wind speeds from damage to non-engineered structures is very subjective and is difficult to defend from various meteorological perspectives. Nothing less than the combined influence and and prestige of the late  Professor Fujita and Allen Pearson, director of NSSFC (National Severe Storm Forecast Center) in 1971 could have brought this much needed system into widespread use. The FPP scale rates the intensity of the tornado, and measured both the path length and the path width.