The Fujita 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
or aerial damage survey, or both; and depending on the circumstances, ground-swirl patterns,
radar tracking, eyewitness testimonies, media reports and damage imagery, as well as photogrammetry
or videogrammetry if motion picture recording is available. The F-Scale was replaced with
the Enhanced Fujita scale in the United States in February 2007.
Background The scale was introduced in 1971 by Tetsuya
Fujita of the University of Chicago, in collaboration with Allen Pearson, head of the National Severe
Storms Forecast Center. The scale was updated in 1973, taking into account path length and
width. In the United States, starting in 1973, tornadoes were rated soon after occurrence.
The Fujita scale was applied retroactively to tornadoes reported between 1950 and 1972
in the National Oceanic and Atmospheric Administration National Tornado Database. Fujita rated tornadoes
from 1916–1992 and Tom Grazulis of The Tornado Project retroactively rated all known significant
tornadoes in the U.S. back to 1880. The Fujita scale was adopted in most areas
outside of Great Britain. In 2007, the Fujita scale was updated, and
the Enhanced Fujita Scale was introduced in the United States. The new scale more accurately
matches wind speeds to the severity of damage caused by the tornado.
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.
Derivation
The original scale as derived by Fujita was a theoretical 13-level scale 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, 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.
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. 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." 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.
Parameters The five categories are listed here, in order
of increasing intensity. Where the relative frequency of tornadoes
is mentioned, it is the relative frequency in the United States, for which the best data
exists. Strong tornadoes occur less often elsewhere in the world. The ratio seems to
be similar except for some areas where it's less such as the United Kingdom, but because
of few overall number of tornadoes there are few strong tornadoes. Areas of eastern India,
and possibly a few other areas do have frequent severe tornadoes; however, data are 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.
*Fujita's initial wind speed estimates have since been found to be highly inaccurate.
See Enhanced Fujita Scale 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.
Rating classifications For purposes such as tornado climatology studies,
Fujita scale ratings may be grouped into classes. 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 went largely unenacted. 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 continue to use the original
Fujita Scale. Environment Canada has begun using the Enhanced Fujita scale in Canada
as of April 18, 2013. See also
Beaufort scale Enhanced Fujita scale
List of F5 and EF5 tornadoes List of tornadoes and tornado outbreaks
Rohn Emergency Scale for measuring the magnitude of any emergency
Saffir–Simpson Hurricane Scale Severe weather terminology
Tornado intensity and damage TORRO scale
Wind engineering References
Notes
Bibliography Marshall, Timothy P.. "Birth of the Fujita
Scale". Storm Track. 24: 6–10. Edwards, Roger; J. G. LaDue; J. T. Ferree;
K. Scharfenberg; C. Maier; W. L. Coulbourne. "Tornado Intensity Estimation: Past, Present,
and Future". Bull. Amer. Meteor. Soc. 94: 641–53. Bibcode:2013BAMS...94..641E. doi:10.1175/BAMS-D-11-00006.1.
External links The Fujita Scale of Tornado Intensity
NOAA National Weather Service Improves Tornado Rating System
Enhanced F Scale for Tornado Damage The Enhanced Fujita Scale
Fujita Scale Enhancement Project Mitigation Assessment Team Report: Midwest
Tornadoes of May 3, 1999 A Guide to F-Scale Damage Assessment
A Guide for Conducting Convective Windstorm Surveys
The Tornado: An Engineering-Oriented Perspective Fujita archival records