A measure of the strength of a tornado, based on wind speed. The scale ranges from F-0 to F-5 ("The Finger of God" according to Bill Paxton in "Twister"). The scale ranges are as follows:

F-0:  40 -  72 mph
F-1:  73 - 112 mph
F-2: 113 - 157 mph
F-3: 158 - 205 mph
F-4: 207 - 260 mph
F-5: 261 - 318 mph

The Fujita-Pearson scale, more commonly known simply as the Fujita Scale, was devised by eminent meteorologists Tetsuya Theodore Fujita and Allen Pearson. It provides a scale against which to measure intensity and damage of tornados. It also used to include classifications based on the width and length of a tornado's damage path, but later meteorological research raised serious questions as to the validity of the path metric and it has been dropped from most uses.

Although the scale is expressed in terms of wind speeds it is actually a measurement of the amount of damage done by a tornado. An extremely strong tornado, even one with F5 force winds, will still be classified as an F0 or an F1 if it occurs in a large open field with no buildings or trees. This also makes it difficult to distinguish between the stronger levels of the scale when the tornado is in a densely populated area. It can be difficult to tell whether damage was directly caused by wind or if it is a secondary effect such as an airborne vehicle striking something.

F0, 40-72 mph, Path .3-.9 miles long, 6-17 yards wide

At 40 mph the wind makes it walking difficult for an adult and impossible for a child, signs break and branches blow off of trees. At 50 driving a compact car is risky, especially in open areas, and large branches snap like small twigs. At 60 driving a high profile vehicle is suicidal while chimneys start to take damage from branches flying through the air like biological shrapnel. At 70 you've made it to hurricane force--shallow-rooted trees in soft soil uproot, driving isn't possible, neither is walking.

F1, 73-112 mph, Path 1-3 miles long, 18-55 yards wide

From 70 to 110 mph the winds rip shingles from roofs, projectiles present a serious danger to anyone caught outside. Mobile homes, damned by the combination of a weak foundation and a wide wind-catching profile, turn over. Someone caught on the road will have their vehicle thrown violently by the wind, the faster they're moving the more likely their vehicle will take flight.

74% of all tornados are F0 or F1 in intensity. 4% of all tornado deaths are in F0 or F1 storms.

F2, 113-157 mph, Path 3-10 miles long, 50-175 yards wide

Mobile homes are toast. Frame houses lose their roofs, ripped completely off by the winds of an F2 tornado. Even large trees begin to come up out of the ground, though they merely fall back to it after doing so. Trains derail, the cars flipped over by the winds. Large branches and debris from damaged building pose significant threats.

F3, 158-206 mph, Path 10-30 miles long, 150-600 yards wide

At this level tornados become truly terrifying. Not just the boxcars, but the engines of trains are thrown about. Older homes or homes not securely anchored to their foundations can become airborne. Whole trees are thrown tremendous distances at very high speeds and large stands of trees are completely uprooted.

25% of all tornados are F2 or F3. 29% of all tornado deaths are caused by these storms.

F4, 207-260 mph, Path 30-100 miles long, .3-1 miles wide

Well-constructed houses are utterly destroyed, turned into missiles. Enormous trees are picked up and thrown about, whole forests treated like the toothpicks in Rain Man. Cars reach speeds their designers never intended--the only time you'll see an AMC Gremlin go 250 is when it's pushed by an F4 tornado.

F5, 261-318 mph, Path 100-300 miles long, 1-3 miles wide

The most devastating class of storm. Large, steel-reinforced concrete structures take significant damage, mostly from small, steel-reinforced concrete structures that have turned into a wall of total devastation travelling at half the speed of sound. Trees that aren't uprooted have all of their bark stripped off, cars can be thrown upwards of 300 to 400 feet, and pretty Kansas girls flit off over the rainbow.

1% of all tornados are F4 or F5 in intensity but they are responsible for 67% of all tornado deaths.


Sources:
http://www.tornadoproject.com/fscale/fscale.htm
http://www.usatoday.com/2000/century/weather/stories/wtorscale.htm
http://www.stormfax.com/fujita.htm

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