| Fujita scale | ||||||
| F0 | F1 | F2 | F3 | F4 | F5 | |
The Fujita scale (F-Scale), or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily 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.
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The scale was introduced in 1971 by Tetsuya 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 the United States, tornadoes from 1973 onward were rated soon after occurrence whereas the scale was applied retroactively to tornado reports from 1950 through 1972 for the National Oceanic and Atmospheric Administration (NOAA) National Tornado Database. Fujita rated tornadoes from 1916–1992 and 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 Previously used in most areas outside of Great Britain, the Fujita scale was superseded in 2007 by the Enhanced Fujita Scale in the United States.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 effectively a damage scale, and the wind speeds associated with the damage listed aren't rigorously verified. The Enhanced Fujita Scale was formulated due to research which suggested that the wind speeds required to inflict damage by intense tornadoes on the Fujita scale are greatly overestimated. A process of expert elicitation with top engineers and meteorologists resulted in the EF scale wind speeds, however, these are biased to United States construction practices. The EF scale also improved damage parameter descriptions.
The original scale as derived by Fujita was a theoretical 13-level scale (F0–F12) designed to smoothly connect the Beaufort scale and the Mach number scale. F1 corresponds to the twelfth level of the Beaufort scale, and F12 corresponds to Mach number 1.0. F0 was placed at a position specifying no damage (approximately the eighth level of the Beaufort scale), in analogy to how the Beaufort's zeroth level specifies little to no wind. 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 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.[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. NOAA 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.
Some sources add level F+", meaning a tornado with winds below 39 mph; on rare occasions tornadoes this weak are observed covering a wide range of path widths and lengths.
The five categories are listed here, in order of increasing intensity.
| Scale | Estimated wind speed*[3] | Relative frequency | Average Damage Path Width (meters) | Potential damage[3] | ||
| mph | km/h | |||||
| F0 | 40–72 | 64–116 | 38.9% | 10–50 | Light damage.
Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged. |
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| F1 | 73–112 | 117–180 | 35.6% | 30–150 | 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. |
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| F2 | 113–157 | 181–253 | 19.4% | 110–250 | Significant damage.
Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; highrise windows broken and blown in; light-object missiles generated. |
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| F3 | 158–206 | 254–332 | 4.9% | 200–500 | Severe damage.
Roofs and some walls torn off well-constructed houses; most trees in forest uprooted; skyscrapers twisted and deformed with massive destruction of exteriors; heavy cars lifted off the ground and thrown. |
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| F4 | 207–260 | 333–418 | 1.1% | 400–900 | Devastating damage.
Well-constructed houses leveled; structures with weak foundations blown away some distance; trains overturned; cars thrown and large missiles generated. Skyscrapers and highrises toppled and destroyed. |
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| F5 | 261–318 | 419–512 | <0.1% | 1100 ~ | 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. |
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*Fujita's initial wind speed estimates have since been found to be highly inaccurate. See Enhanced Fujita Scale
NB: Path widths are averages of tornadoes of their Fujita rating and are approximate values; tornado intensity should not be assumed by the size of the damage path. One F5 tornado had a damage path just 90 meters wide at one point.
Since the Fujita scale is based on the severity of damage resulting from high winds, an F6 tornado is a purely theoretical construct. Property damage cannot exceed total destruction, which constitutes an F5. (A tornado with windspeeds greater than 319 miles per hour is theoretically possible, and the 1999 Oklahoma City tornado may have been such an event. However, no such wind speed has ever been recorded and that measurement was not near ground level.)
| F0 | F1 | F2 | F3 | F4 | F5 |
|---|---|---|---|---|---|
| Weak | Strong | Violent | |||
| Significant | |||||
| Intense | |||||
For purposes such as tornado climatology studies, Fujita scale ratings may be grouped into classes.[4][5][6]
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.[7] 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,[8] 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 went largely unenacted.[9]
In the USA and some other countries, 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 estimate for wind speeds, and sets no upper limit on the wind speeds for the strongest level, EF5. Several countries, including Canada and France, continue to use the original Fujita Scale.