Tornadoes are classified using a scale known as the "F" or Fujita scale. The scale is named after its developer, Dr. Ted Fujita. Unlike hurricanes which are classified throughout their development, tornadoes are not given a classification on this scale until one has moved through an area and caused damage. This is due to the fact that measuring wind speeds directly within a tornado remains nearly impossible. Meteorologists survey areas impacted by tornadoes after they have moved through and give them a classification on the Fujita scale based on the observed damage. As shown below, the scale ranges from F0 to F5, with F0 being the weakest and F5 the strongest tornadoes. An F6, while possible, is believed to highly unlikely to ever occur.
F-0 Gale tornado (40-72 mph): Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.
F-1 Moderate tornado (73-112 mph): The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.
F-2 Significant tornado (113-157 mph): Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.
F-4 Devastating tornado (207-260 mph): Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.
F-5 Incredible tornado (261-318 mph): Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel-reinforced concrete structures badly damaged.
F-6 Inconceivable tornado (319-379 mph): These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F-4 and F-5 wind that would surround the F-6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F-6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies