TORNADOES—Know the threat and be prepared.
The Fujita Scale(also known as the Fujita-Pearson Scale)

    The Fujita Scale(also known as the Fujita-Pearson Scale) may not be a perfect system for linking damage to wind speed, but it had distinct advantages over what had gone on before its inception. And it 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. The Fujita part of the scale is as follows:
F-Scale Number Intensity Phrase Wind Speed Type of Damage Done
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 fores 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 re-inforced concrete structures badly damaged.


MYTH ... Highway overpasses are a safe place to shelter if you are on the road when you see a tornado coming.
The truth is, any time you deliberately put yourself above ground level during a tornado, you are putting yourself in harm's way.

MYTH ... Opening windows to equalize air pressure will save a roof, or even a home, from destruction by a tornado.
The idea that moving one thin pane of glass is going to protect a roof or house from one of the most violent natural forces on the planet has a certain absurdity about it. It is probably born of wishful thinking and faulty logic, stemming from the need to do something ... anything. In reality, opening windows is a dangerous and useless waste of time, and could actually be harmful to the house.

MYTH ... Some towns are “protected.”
That logic disregards some very basic ideas. It ignores the likely possibility that rivers, ridges, and valleys have little or no effect on mature tornadoes. Tornadoes have passed seemingly unaffected over mountain ridges 3,000 feet high. Dozens have crossed the Mississippi River, from Minnesota to Louisiana. Both sides of the river, at the confluence of the Mississippi and Missouri Rivers, near St. Louis, have seen devastating tornadoes.