"The introduction of anemometers led to the necessity for a scale of equivalents between Beaufort numbers estimated by experienced observers and the velocity of the wind in miles per hour. Experiments showed that the relation could be expressed approximately by the equation v = 1.87 x B 3/2 mph, where B is the corresponding Beaufort number"At a meeting in 1926 the International Meteorological Committee accepted Dr. George Simpson’s table of equivalents [of which I give a part here]
|B0||0 - 0.5||0 – 1|
|B1||0.6 - 1.7||2 - 3|
|B2||1.8 – 3.3||4 – 7|
|B8||15.3 – 18.2||34 - 40|
|B10||25.2 – 29.0||57 - 65|
In the absence of actual wind-speed measuring instrumentation "the direct estimation of tornado wind-speeds is best achieved by studying the evidence of certain kinds of structural damage (as for instance to engineered structures), or by analysing ciné-film or video film sequences of entrained debris, the flight and impact of projected missiles, photographs of the shapes of funnel clouds, and so on. Good evidence like this is not often available, and is only acquired gradually during many years of patient data accumulation. Hence, in order to facilitate a rapid understanding of tornadic strengths in day-to-day examples, and to permit meaningful intercomparisons to be readily drawn between them, the straightforward scale of intensities devised by TORRO has been used in the basic British/European tornado data bank. The intensity numbers on this scale represent wind-speed bands pertaining to their damage potential . The assignment of an intensity number simplifies discussion of a tornado’s most significant attribute, its maximum known strength; at the same time, it greatly aids information retrieval. Of the many uses to which it can be put, one of the most important is certainly the statistical analysis of past incidents within a selected region to establish tornado-risk probabilities at different levels of intensity."It is important to note the straightforward relationship between T–Scale and B scale, namely:
T.T. Fujita set his tornado force 1 (more precisely F1.0) to equal minimum hurricane speed, i.e. he chose not the normal Beaufort mean hurricane force 12.0 but what amounts to B11.53 instead (about 73 mph). This peculiarity ensured that there could never be an exact equivalence between F and B scales.
Having noted the existence of the Beaufort scale as being useful for speeds below hurricane speed, Dr. Fujita reflected that, in case the highest wind speeds in tornadoes might be found to approach Mach 1 (738 mph or 330 metres per second), he should allow for that when proposing his tornado damage scale.
So he set out a three-part graph running from 0 mph to beyond 750 mph, in which he had to forge a link between Beaufort at one end and the Mach scale at the high end, where he chose F12 to be Mach 1. This also had the effect of making F0 equal to B7.7 (not B8)
He achieved the link by interposing a third section to a special design that nonetheless followed the usual 3/2 power law, but in order to make the mismatch work Dr. Fujita assigned arbitrary constants to his new line (Fujita 1973; Fujita and Pearson 1973)).
A major consequence is that there are no exact numerical relations between the Beaufort and Fujita Scales:
Abbey, R. F. (1976). Page 187 of Risk probabilities associated with tornado windspeeds, in Proc. Symposium on tornadoes, Lubbock, Texas, June 1976.
Fujita (1973). Tornadoes around the world. Weatherwise. 26, 56-62.
Fujita, T.T. and Pearson, A.D. (1973) Results of FPP classification of 1971 and 1972 tornadoes. Preprints 8th Conference on Severe Local Storms, Denver. October 1973. Pp. 142-145
Meaden, G.T. (1975-76). Tornadoes in Britain: their intensities and distribution in time and space. J. Meteorology, 1, 242-251 (based on a lecture to the Royal Meteorological Society in 1975).
Meaden, GT (1985) A study of tornadoes in Britain, with assessments of the general tornado risk potential and the specific risk potential at particular regional sites. Prepared at the request of HM Nuclear Installations Inspectorate Health and Safety Executive.