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Fujita scale

The Fujita scale (F-Scale), or Fujita-Pearson scale, is a scale for rating tornado intensity, based on the damage tornadoes inflict on human-built structures and vegetation. The official Fujita scale category is determined by meteorologists (and engineers) after a ground and/or aerial damage survey; and depending on the circumstances, ground-swirl patterns (cycloidal marks), radar tracking, eyewitness testimonies, media reports and damage imagery, as well as photogrammetry/videogrammetry if motion picture recording is available.

Background

The scale was introduced in 1971 by Tetsuya "Ted" Fujita of the University of Chicago who developed the scale together with Allen Pearson (path length and width additions in 1973), head of the National Severe Storms Forecast Center (predecessor to the Storm Prediction Center) in Kansas City, Missouri. The scale was applied retroactively to tornado reports from 1950 onward for the National Oceanic and Atmospheric Administration National Tornado Database in the United States, and occasionally to earlier infamous tornadoes. Tom Grazulis of The Tornado Project also rated all known significant tornadoes (F2-F5 or causing a fatality) in the U.S. back to 1880. Previously used in most areas outside of Great Britain, it was superseded in 2007 by the Enhanced Fujita Scale in the United States.

Though each damage level is associated with a wind speed, the Fujita scale is a damage scale, and the wind speeds associated with the damage listed are unverified. The Enhanced Fujita Scale was formulated due to research which suggested that wind speeds for strong tornadoes on the Fujita scale are greatly overestimated. However, being determined by expert elicitation with top engineers and meteorologists, the EF scale wind speeds remain as educated guesses, and are also biased to United States construction practices.

Derivation

The original scale as derived by Fujita was a 13-level scale (F0-F12) designed to smoothly connect the Beaufort scale and the Mach number scale. The gap between F0 and F1 corresponds to the eighth and twelfth levels of the Beaufort scale, "violent storm" and "hurricane" respectively. On the original scale, the wind speeds for F11 and F12 corresponded to Mach numbers 0.9 and 1.0 respectively. This provided a smooth relationship among the three scales. From these wind speed numbers, qualitative descriptions of damage were made for each category of the Fujita scale, and then these descriptions were used to classify tornadoes. The diagram on the right illustrates the relationship between the Beaufort, Fujita, and Mach number scales.

At the time Fujita derived the scale, little information was available on damage caused by wind, so the original scale presented little more than educated guesses at wind speed ranges for specific tiers of damage. Fujita intended that only F0-F5 be used in practice, as this covered all possible levels of damage to frame homes as well as the expected estimated bounds of wind speeds. He did, however, add a description for F6, which he phrased as "inconceivable tornado", to allow for wind speeds exceeding F5 and for possible future advancements in damage analysis which might show it.

Furthermore, the original wind speed numbers have since been found to be higher than the actual wind speeds required to incur the damage described at each category. The error manifests itself to an increasing degree as the category increases, especially in the range of F3 through F5. The National Oceanic and Atmospheric Administration notes that …precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar-looking damage from place to place—even from building to building. Without a thorough engineering analysis of tornado damage in any event, the actual wind speeds needed to cause that damage are unknown. Since then, the Enhanced Fujita Scale has been created using better wind estimates by engineers and meteorologists.

Parameters

The six categories are listed here, in order of increasing intensity. Note:

When the relative frequency of tornadoes is mentioned, it is the relative frequency in the United States. Frequencies of strong tornadoes (F2 or greater) are significantly less elsewhere in the world. Parts of southern Canada, Bangladesh and adjacent areas of eastern India, and possibly a few other areas do have frequent severe tornadoes, however data is scarce and statistics in these countries have not been studied thoroughly.
The rating of any given tornado is of the most severe damage to any well-built frame home or comparable level of damage from engineering analysis of other damage.
The F6 level, although present in Dr. Ted Fujita's original wind scale, was not intended for use, is not an official damage level and is not used to rate tornadoes. There is, by definition, no such thing as an 'F6' tornado.

Scale	Estimated wind speed^*		Relative frequency	Potential damage
Scale	mph	km/h	Relative frequency	Potential damage
F0	40–72	64–116	38.9%	Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.
F1	73–112	117–180	35.6%	Moderate damage. 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	113–157	181–253	19.4%	Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated.
F3	158–206	254–332	4.9%	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.
F4	207–260	333–418	1.1%	Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
F5	261–318	419–512	Less than 0.1%	Incredible damage. Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 m (109 yd); trees debarked; steel reinforced concrete structures badly damaged; incredible phenomena will occur.

*Fujita's initial wind speed estimates have since been found to be highly inaccurate. See Enhanced Fujita Scale

Decommission in the US

The Fujita scale, introduced in 1971 as a means to differentiate tornado intensity and path area, assigned wind speeds to damage that were, at best, educated guesses. Fujita and others recognized this immediately and intensive engineering analysis was conducted through the rest of the 1970s. This research, as well as subsequent research, showed that tornado wind speeds required to inflict the described damage were actually much lower than the F-scale indicated, particularly for the upper categories. Also, although the scale gave general descriptions for the type of damage a tornado could cause, it gave little leeway for strength of construction and other factors that might cause a building to receive higher damage at lower wind speeds. Fujita tried to address these problems somewhat in 1992 with the Modified Fujita Scale, but by then he was semi-retired and the National Weather Service was not in a position for the undertaking of updating to an entirely new scale, so it was a minor step.

In the USA only, on February 1, 2007, the Fujita scale was decommissioned in favor of what these scientists believe is a more accurate Enhanced Fujita Scale, which replaces it. The EF Scale is thought to improve on the F-scale on many counts—it accounts for different degrees of damage that occur with different types of structures, both man-made and natural. The expanded and refined damage indicators and degrees of damage standardize what was somewhat ambiguous. It also is thought to provide a much better estimates for wind speeds, and sets no upper limit on the wind speeds for the strongest level, EF5.

The original Fujita scale is still used in most of the rest of the world, except where the TORRO scale is used.