Weather: Tipping the scales for tornado and hailstorm

How do you measure the force of something that is strong enough to blow away your recording equipment? The most violent weather is often best estimated by the damage it causes.

Our coverage of the recent tornado in Selsey left one reader perplexed. Andrew Simpson, of Shepherds Bush, London, wrote: "I am sure you can imagine my consternation when two articles, written expertly about the subject of tornadoes, were so abjectly inconsistent. On page three of The Independent on 9 January, the article insists that `Their [the tornadoes'] intensity is measured against the seven-point Fujita scale, ranging from F0 to F6'; however on page 26 of the Eye, the article categorically states that, `there is a scale - called the Torro scale - to grade the intensity of tornadoes' and the full Torro scale as described is a 12-point description of severity. Could you please explain this apparent inconsistency and identify, for those pregnant pauses at cocktail parties, which scale should be used?"

That's a good question, so first let us set your mind at rest over the question of inconsistency. Fujita (or Fujita-Pearson to give it its full name) and Torro are simply two different scales by which to measure tornado strength. The first was developed by Dr T Theodore Fujita and Allen Pearson in America in 1971; the second was developed independently by Dr G Terence Meaden in Britain in 1972. Both are based on the principle of using the damage caused by a tornado to assess its wind speed. Since British tornadoes tend to be milder than American ones, it is not surprising that the scale developed here has more points, to enable greater gradation of moderate tornadoes.

On the general problem of measuring high wind speeds, it is worth mentioning a highly sceptical comment in Robert Scott's Elementary Meteorology of 1887. Mentioning an account of a 120mph wind supposedly observed by a meteorologist named Rochon, he says: "We are not informed of the effect of the wind which Rochon observed; if it did more than throw down buildings it must have been hard to measure its force." (This comment follows an account of a 110mph wind which "threw down buildings".)

The direct measurement of high winds remains a problem for anyone who does not want his anemometer blown away, so tornadoes are still generally measured by their effects.

When chatting about the Torro tornado intensity scale at cocktail parties, any lull in the conversation may be filled by a quick change of subject to the Torro Hailstorm Intensity Scale, which answers a question posed in this column a week or so ago. the Hailstorm Scale, devised by Jonathan Webb in 1986, goes from H0 (hail the size of peas, but no damage caused) to H10 (wooden houses destroyed; risk of fatal injury to unprotected persons), but the interesting thing is the accompanying scale of hailstone sizes, from 1 (5-10mm; pea); 2 (11-15mm; mothball, bean, hazelnut); via 5 (31-45mm; chestnut, pigeon's egg, golf-ball, table-tennis ball, squash ball) and 6 (46-60mm; hen's egg, small peach, small apple, billiard ball) up to 9 (101-125mm; melon) and 10 (over 125mm; coconut).

All this time, I had thought "hailstones the size of hen's eggs" was vague journalistic shorthand. Now I know it is a point on a scientific scale.