Science Fair Project Encyclopedia
The Fujita scale rates a tornado's intensity by the damage it inflicts on human-built structures. It was introduced in 1971 by T. Theodore Fujita of the University of Chicago who developed the scale together with Allan Pearson , head of the Forecast Center in Kansas City, Missouri.
Fujita scale measurements are issued after a tornado has passed through an area, not while it is on the ground. It is possible, but difficult, to make educated guesses as to the probable F catagory while a tornado is on the ground. The official Fujita scale measurement is determined after scientists examine radar tracking, eye-witness testimonies, and the damage caused by the tornado.
The six categories are, in order of increasing intensity:
|Category F0||Wind speed||<73 mi/h||<115 km/h||Relative frequency||29%|
|Potential damage||Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.|
|Category F1||Wind speed||73–112 mi/h||116–180 km/h||Relative frequency||40%|
|Potential damage||Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.|
|Category F2||Wind speed||113–157 mi/h||181–250 km/h||Relative frequency||24%|
|Potential damage||Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.|
|Category F3||Wind speed||158–206 mi/h||251–330 km/h||Relative frequency||6%|
|Potential damage||Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.|
|Category F4||Wind speed||207–260 mi/h||331–415 km/h||Relative frequency||2%|
|Potential damage||Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.|
|Category F5||Wind speed||261–318 mi/h||416–510 km/h||Relative frequency||<1%|
|Potential damage||Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur. Twister refers to an F5 as "The Finger of God", referring to the mental imagery of God dragging a finger across the Earth, destroying everything in its path.|
Reliability of measurements
Note that wind speeds empirically derived from engineering data from damage surveys can only be approximate, but highly reliable. On the other hand, wind speeds verified with high resolution doppler radar (better than NEXRAD), return accurate wind speeds and Fujita scale ratings. Tornadoes with an intensity greater than F5 are commonly regarded as F5. This is mostly due to the fact that the theoretical maximum potential of variables of the atmosphere may have an upper limit at F5. Also note that a hypothetical F12, the level at which the F-scale peaks, would correspond to Mach 1, i.e. the speed of sound.
It is suspected that an F6 or greater tornado would be indistinguishable from F5 because significant damage would be done by the sides before the F6 center met objects, and that only a narrow cone would exceed F5, and likely for a limited time. A tiny proportion of tornados have any part near the top end of the F5 category. In theory, F6 would correspond to 319–379 mi/h (511–610 km/h). It should be noted that all tornados above F3, combined, are 2% of the total.
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