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B. Geerts |
1/'99 |
It is difficult to quantify the intensity of tornadoes. To measure the intensity of tropical cyclones, dedicated aircraft fly reconnaissance missions into the eye of these storms. However tornadoes are too small and too intense for in situ wind or pressure measurements. Bigger tornadoes are usually stronger, but the visible size of the funnel also depends on ambient humidity, and a camera record only exists for a fraction of all tornadoes.
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In 1971 Tetsuya Theodore Fujita, a professor at the University of Chicago, came up with a system to rank tornadoes according to how much damage they cause. He contrived the categories by connecting the Beaufort wind scale (Table 14.1 in the book) with the speed of sound (1 Mach) in 12 steps (Fig 1). Twelve simply because the Beaufort scale had 12 steps as well. For each category he then estimated how strong the wind must have been to cause some observed damage. The resulting classification (Table 1) became known as the Fujita scale, sometimes referred to as the Fujita-Pearson Tornado Intensity Scale, since Allen Pearson of the Forecast center in Kansas City did much of the groundwork to identify tornado damage (1).
Fig 1 (right). Design of the Fujita tornado intensity scale, beween the inadequate Beaufort scale and the speed of sound (Source: T.T. Fujita). |
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Table 1. The Fujita-Pearson Tornado Intensity Scale, plus an approximate intensity frequency distribution in the United States.
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classification |
max sustained wind speed |
damage |
approx percentage of all tornadoes in the USA |
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|
(km/h) |
(m/s) |
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F0 (weak) |
64-116 |
18-32 |
light |
83% |
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F1 (strong) |
117-180 |
32-50 |
moderate |
11% |
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F2 (strong) |
181-253 |
50-70 |
considerable |
4% |
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F3 (violent) |
254-332 |
70-92 |
severe |
1.8% |
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F4 (violent) |
333-419 |
92-116 |
devastating |
0.9% |
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F5 (violent) |
420-512 |
116-142 |
unbelievable |
0.4% |
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513-612 |
142-170 |
inconceivable! |
?? (none yet) |
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Clearly the vast majority of tornadoes, and almost all unreported tornadoes, are weak (F0). Only 3% of the tornadoes is violent in the USA; elsewhere this fraction is even lower. The maximum wind speeds of tornadoes are not expected to reach the F6 wind speeds. Observational and theoretical evidence suggest that the strongest tornadoes on Earth are of F5 strength.
A comparison between the Fujita tornado intensity scale and the Saffir-Simpson hurricane damage potential scale (Table 13.2) shows the following. The maximum sustained winds in tropical cyclone classes 1-2 are those of a F0 tornado. A F1 and a weak F2 tornado is most likely to match a SS3 and SS4 hurricane, respectively. And a SS5 hurricane has the wind of a strong F2 or a F3 tornado. Hypercanes may have winds as strong as a solid F5 tornado.
One should be careful in drawing conclusions from this comparison between very different phenomena. A hurricane circulation is much larger than that of a tornado. A hurricane is a mature mode of organization of a cluster of thunderstorms, while a tornado is a short-lived vortex spun up by a convective updraft. Tornadoes do sometimes occur in the eyewall or spiral bands of a tropical cyclone, especially at landfall, and in the left forward quadrant (right in the northern hemisphere) of the storm.
Reference
(1) Fujita, T.T. 1981. Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38, 1511-1534.
