Fujita or F Scale for wind velocity

In 1971, T. T. Fujita proposed a wind scale to cover the range from force 12 at the top of the Beaufort scale to Mach 1, the speed of sound in air, in 12 equal steps.¹ The equivalent on the Fujita scale or F-scale of any wind speed M (in meters per second) can be found from the equation

F equals the 1.5th root of the fraction M over 6.3. minus 2

In practice, however, only whole numbers are used, and the wind velocity is estimated from the damage caused to structures, observable in the aftermath, in which respect the Fujita scale resembles the Mercalli Scale for earthquake intensity.

T. T. Fujita.
Proposed characterization of tornadoes and hurricanes by area and intensity.
Satellite and Mesometeorology Research Project Research Report 91.
The University of Chicago, 1971.

T. T. Fujita.
F-Scale classification of 1971 Tornadoes.
SMRP Research Report 100.
The University of Chicago, 1972.

1. T. T. Fujita.
Tornadoes and downbursts in the context of generalized planetary scales.
Journal of the Atmospheric Sciences, volume 38, number 8 (August 1981).

In the United States, since 1 February 2007 official estimates of tornado wind speeds have been made using the Enhanced Fujita Scale instead of the Fujita Scale. Enhanced F-Scale estimates are written, for example, “EF4”, while an estimate on the Fujita scale would be writtten “F4”.

Fujita scale for damaging wind
F
Number
Wind speed Description
meters
per
second
miles
per
hour
knots
F0 18-32 40-72 35-62 Light damage. Some damage to chimneys; break branches off trees; push over shallow-rooted trees; damage sign boards.
F1 33–49 73–112 63–97 Moderate damage. The lower limit is the beginning of hurricane wind speed. Peel surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads.
F2 50–69 113–157 98–136 Considerable damage. Roofs torn off frame houses, mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated.
F3 70–92 158–206 137–179 Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown.
F4 93–116 207–260 180–226 Devastating damage. Well-constructed houses leveled; structure with weak foundation blown off some distance; cars thrown and large missiles generated.
F5 117–142 261–318 227–276 Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile-sized missiles fly through the air in excess of 100 meters; trees debarked; incredible phenomenon will occur. [Steel-reinforced concrete structures damaged.]
F6–
F12
142–
Mach 1
319–Mach 1 277– Mach 1 Not expected to occur on Earth.

When the Fujita scale is applied to tornadoes, the force numbers have approximately these meanings:

Force Description
F0 a gale tornado
F1 a moderate tornado
F2 a significant tornado
F3 a severe tornado
F4 a devastating tornado
F5 an incredible tornado

Fujita-Pearson scale

In January 1972, Fujita sent a copy of SMRP Paper 91 to every National Weather Service office, to the attention of the Meteorologist-in-Charge. Attached was a letter coauthored by Allen D. Pearson, then the Director of the National Severe Storms Forecast Center. The letter asked the local service to use the Fujita-Pearson Scale in describing tornadoes in their area.

T. T. Fujita and A. D. Pearson.
Results of FPP classification of 1971 and 1972 tornadoes.
Preprints, Eighth Conference on Severe Local Storms, Denver, CO, American Meteorological Society, pages 142–145. (1973).

Pearson's addition was the length and average width of the damage swath. In other words, they were an additional observation of damage, but did not estimate wind speeds.

F number Pearson Length
in miles
(kilometers)
Pearson Width
in yards
(meters)
0 0.3 – 0.9
(< 1.6)
6 - 17
(<16)
1 1.0 – 3.1
(1.6 – 5.0)
18 – 55
(16–50)
2 3.2 – 9.9
(5.1 – 15.9)
56 – 175
(51 – 160)
3 10 – 31
(16 – 50)
176 – 556
(161 – 508)
4 32 – 99
(51 – 159)
0.3 – 0.9 miles
(509 – 1448)
5 100 – 315
(160 – 507)
1.0 – 3.1 miles
(1449 – 4989)
6 316 – 999 3.2 – 9.9 miles

A reading on the Fujita-Pearson scale was reported as three comma-separated digits, the first Fujita's wind-from-damage scale, the second the Pearson length, and the third the Pearson width. For example, 2,3,3. The scale was often called the FPP scale. The PP values proved to be not very useful.

They were essentially logarithmic categorizations of the observed path length and the mean path width. For instance a tornado with PL= 0 had a path that ranged between 0.3 and 1 mile long. In contrast, a PL=5 tornado had a path length between 100 and 315 miles long. There simply was too much resolution in the lower numbers and too little resolution at higher numbers to adequately differentiate the track dimensions of tornadoes. Because of this, the PL and PW portions of the scale are seldom used anymore.

Daniel McCarthy, Joseph Schaefer and Roger Edwards.
What Are We Doing with (or to) the F-Scale?
American Meteorological Society, 23rd Conference on Severe Local Storms, St. Louis, (2006).
Available online at www.spc.noaa.gov/publications/mccarthy/slsc23.pdf

The disuse of the Fujita-Pearson scale is evident in Google's ngram viewer. In most of the later instances of use, what is described is not the FPP scale but simply the Fujita (or EF) scale.

Photo of tornado

© copyright istockphotocom/Clint Spencer

An F3 tornado.

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