|0 --- Calm||less than 1 mph||Smoke rises vertically|
|1 --- Light air||1 - 3 mph||Smoke drifts with air, weather vanes inactive|
|2 --- Light breeze||4 - 7 mph||Weather vanes active, wind felt on face, leaves rustle|
|3 --- Gentle breeze||8 - 12 mph||Leaves & small twigs move, light flags extend|
|4 --- Moderate breeze||13 - 18 mph||Small branches sway, dust & loose paper blows about|
|5 --- Fresh breeze||19 - 24 mph||Small trees sway, waves break on inland waters|
|6 --- Strong breeze||25 - 31 mph||Lagre branches sway, umbrellas difficult to use|
|7 --- Moderate gale||32 - 38 mph||Whole trees sway, difficult to walk against wind|
|8 --- Fresh gale||39 - 46 mph||Twigs broken off trees, walking against wind very difficult|
|9 --- Strong gale||47 - 54 mph||Slight damage to buildings, shingles blown off roof|
|10 -- Whole gale||55 - 63 mph||Trees uprooted, considerable damage to buildings|
|11 -- Storm||64 - 73 mph||Widespread damage, very rare occurrence|
|12 -- Hurricane||over 73 mph||Violent destruction|
Tornadoes are measured using a scale that measures the amount of damage the tornado causes. The scale is known as the "Fujita Tornado Intensity Scale":
F0 (Gale tornado) 40-72 mph
Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.
F1 (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.
F2 (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.
F3 (Severe tornado) 158-206 mph
Roof and some walls torn off well constructed houses; trains overturned; most trees in forest uprooted
F4 (Devastating tornado) 207-260 mph
Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.
F5 (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.
F6 (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
[quoted from Tornado Project]
Between 1950 and 1994, 74% of all of the tornadoes that touched down in the United States were "weak" (F0 or F1), 25% were "strong" (F2 or F3), and only 1% were "violent" (F4 or F5).
Note: Above table may contain errors and should be used only at your own risk. The abbreviation "mph" signifies "miles per hour."