Everything about The Fujita Scale totally explained
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Great Britain, it was superseded in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 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. 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. 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's 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, wasn't intended for use, isn't an official damage level and isn't used to rate tornadoes. There is, by definition, no such thing as an 'F6' tornado. 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 wasn't 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.
Further Information
Get more info on 'Fujita Scale'.
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