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 was determined by meteorologists (and engineers) after examining damage, ground-swirl patterns, radar tracking, eyewitness testimonies, media reports and damage imagery, as well as photogrammetry/videogrammetry if video was available.

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 in the United States, and occasionally to earlier infamous tornadoes. Previously used in most areas outside of Great Britain, it has since been superseded by the Enhanced Fujita Scale in the United States.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, the EF scale wind speeds remain little more than educated guesses.

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 eleventh 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 between 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.^[1] The diagram on the right illustrates the relationship between the Beaufort, Fujita, and Mach number scales.

At the time Fujita derived the scale, little data 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. After the scale was derived, it was soon realized that F7-F12 levels did not correspond to actual tornado damage. Later, the National Weather Service decided not to use the F6 level either, as identifying the damage caused by a tornado stronger than F5 would be next to impossible. ^[2]

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. ^[2] 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. 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. Canada, Bangladesh and adjacent areas of eastern India also have frequent severe tornadoes, however 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, while present in Dr. Ted Fujita's original wind scale, is not an official damage level and is not used to rate tornadoes. There is, by definition, no such thing as an 'F6' tornado.^[2]

Decommission

Main article: Enhanced Fujita Scale

Though the Fujita scale was a scientific improvement over estimates of tornado strength from earlier in the century, the wind speeds contained in the scale were, at best, educated guesses. Research, especially in the 1980s and 1990s, showed that tornado wind speeds were actually much lower than the F-scale indicated. Also, while the scale gave general descriptions for the type of damage each tornado could cause, it gave no leeway for strength of construction and other factors that might cause a building to receive higher damage at lower wind speeds.

On February 1, 2007, the Fujita scale was decommissioned in favor of the more accurate Enhanced Fujita Scale, which replaces it. The EF Scale improved 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. It also provides much better estimates for wind speeds, and sets no upper limit on the wind speeds for the strongest level, EF5.

See also

• Enhanced Fujita Scale
• TORRO scale
• Beaufort scale
• Saffir-Simpson Hurricane Scale
• List of tornadoes and tornado outbreaks
• List of F5 tornadoes
• Severe weather terminology

References

1. ^ http://www.spc.noaa.gov/efscale/
2. ^ ^{a b c} Tornado FAQ. Storm Prediction Center. Site accessed June 27, 2006.

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