The Fujita Scale

F-Scale Number
Intensity Phrase
Wind Speed
Type of Damage Done
Gale tornado
40-72 mph
Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.
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.
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. 
Severe tornado
158-206 mph
Roof and some walls torn off well constructed houses; trains overturned; most trees in forest uprooted
Devastating tornado
207-260 mph
Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.
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 re-inforced concrete structures badly damaged.
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 F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 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
NOTE: The Fujita Scale is based on damage, not the appearance of the funnel. 1