Fujita Scale for tornadoes
The Fujita Scale is named for Dr. T. T. (Ted) Fujita, who made the first systematic study of tornado forces. Since, until recently, it was very difficult to actually measure wind speeds in a tornado, he devised a classification scheme based on the damage or other effects caused by the winds. The following chart gives the wind speeds and damage summary for the various levels.
A single tornado generally changes strength during its life, so you might have F2 damage at one point, F3 at another, and only F1 at a third, all from the same tornado. Also, many tornadoes occur in unpopulated areas with few structures or trees (i.e. farm fields). Estimation of wind speeds in areas such as this is much more difficult.
Finally, everyone agrees that a tornado cannot really be quantified by a single number, but a scale like this at least gives you a starting point to work with. There is also some contention that there are qualitative differences between the common or garden variety F1, F2, F3 tornadoes and the bigger F4 and F5 tornadoes. The larger tornadoes often have multiple suction vortices (perhaps?, after all, nobody has really gotten good fine structure measurements of any tornado, much less a big F4 or F5).
F- Doubtful Tornado
less than 40 mph
40 mph speed corresponds to Beaufort 8 or "Fresh Gale". Beaufort specification for use on land is "Breaks twigs off trees". Little damage is expected.
F 0 Very Weak Tornado
40-72 mph
This speed range corresponds to Beaufort 9 through 11. Some damage to chimneys or TV antennae; breaks branches off trees; pushes over shallow-rooted trees; old trees with hollow inside break or fall; sign boards are damaged.
F1 Weak Tornado
73-112 mph
73 mph is the beginning of huricane windspeeds or Beaufort 12. Peels surface off roofs; windows broken; trailer houses pushed or overturned; trees on soft ground uprooted; some trees snapped; moving autos pushed off the road.
F2 Strong Tornado
113-157 mph 100 - 200 yds wide
Roof torn off frame houses leaving strong upright walls standing; weak structure or outbuildings demolished; trailer houses demolished; railroad boxcars pushed over, large trees snapped or uprooted; light-object missiles generated; cars blown off highway; block structures and walls badly damaged.
F3 Severe Tornado
158-206 mph - 200yds - 1/4 mi across
Roofs and some walls torn off well-constructed fame houses; some rural buildings completely demolished or flattened; trains overturned; steel framed hangar-warehouse type structures torn; cars lifted off ground and may roll some distance; most trees in a forest uprooted, snapped, or leveled; block structures often leveled.
F4 Devastating Tornado
207-260 mph 1/4 - 1 mile across
Well constructed fame houses leveled, leaving piles of debris; structure with weak foundation lifted, torn, and blown off some distance; trees debarked by small flying debris; sandy soil eroded and gravels fly in high winds; cars thrown some distances or rolled considerable distances finally to disintegrate; large missiles generated.
F5 Incredible Tornado
261-318 mph 1/2-2 miles across
Strong frame houses lifted clear off foundation and carried consderable distance to disntegrate; steel reinforced concrete structures badly damaged; automobile sized missiles fly through the distance of 100 yds or more; trees debarked completely; incredible phenomena can occur.
F6 Inconceivable Tornado
319 mph to Mach 1
The effects are inconceivable. There is reason to believe that there is a "thermodynamic speed limit", and winds faster than 300 mph cannot occur in a tornado.
