and the diffuse nature of the arcs, it was not possible to decide in which order the colours were placed in these bows.

The interpretation of this observation in terms of the known theoretical and laboratory behaviour of raindrops and rainbows is some- what difficult. The excellent paper by Walker (1976) outlines the production of multiple rain- bows from a single water drop and provides angular diameters for bows up to the 20th order. The statement in this paper that no verified observations had been made of any but the first two rainbows elicited several responses which were subsequently reported by Walker (1978). In one of these, J . R. Prescott reported seeing a rainbow in conditions similar to the present observation, against dark clouds above the Sun on the horizon. Walker comments that this might have been the 3rd order rainbow but. since no angular extent or radius was given, it is not obvious whether this was so or whether Prescott's observation was closer to the present one. More recently, Pedgley (1986) reported the definite observation from Nairobi of a ter- tiary bow under similar conditions, while look- ing towards the Sun into an intense shower. In Walker's 1976 paper. the 3rd and 4th order rainbows are shown to be close together in the sky and significantly broader than the primary or secondary bows, but are expected at angles of about 40 and 45 degrees from the solar direction. respectively. The 8th order bow is predicted to be at about 20 degrees from the Sun but can be expected to be broad and very faint, even against a dark background.

Thus, the present observation is not easily fitted into the presently understood scheme of rainbow formation. It is unlikely that a gross error was made in the estimate of angular extent of the observed bows. The closeness to the solar direction could indicate an inter-

pretation favouring the effect of diffraction or a coronal halo. but these are usually seen through relatively uniform light cloud, rather than the present situation where sunlight was passing through heavy rain. There was no mist upon the car windshield to cause such an intense halo.

It is unfortunate that photography of this magnificent display was not possible. It is hoped that the present report might alert others to the possibility of such displays under similar atmospheric conditioins. Department of Physics and T. A. CLARK Astronomy, University of Calgary

REFERENCES

Pedgley. D. E. (1986) A tertiary rainbow. Weather, 41, p. 401

Walker, J. (1976) Multiple rainbows from single drops of water and other liquids. Amer. J. Phys.. 44, pp. 421-433 - (1978) The amateur scientist. Scientific

American, 239-4, pp. 184-185

St. Swithin Expresses Uncertainty Now that forecasters are inclined to pre-

dict more in terms of probabilities. may I sug- gest that the best version of St. Swithin, worth keeping in mind, is the less well known rhyme quoted in the Sunday Telegraph by Philip Eden (12 July 1992):

"In this month is St. Swithin's Day On which if that it rain they say, Full forty days after it will More or less some rain distil."

Budleigh Salterton. Devon

R. B. M. LEVICK

Viewpoint: BBC RADIO WEATHER FORECASTS

For several months now, Radio 4 weather forecasts have been coming from the BBC Weather Centre (at Television Centre), these being presented by the Met. Office-employed staff stationed there. I write to offer some comments, but 1 must declare an interest, in that I was, for a short time, a member of the erstwhile London Weather Centre radio weather presentation team. However, I do not claim any special expertise, and these comments are pre- sented more as an interested radio listener, than with any professional axe to grind.

There has been a perceptible change in

style. I find it sometimes difficult to visualise the information being presented, which does not always follow a logical path. I offer the following guidelines for the national weather forecasts presented on Radio 4, and perhaps this will stimulate some correspondence on the subject:

(a) The forecast should start with a brief introduction, with a reference to the syn- optic situation if it would help the listener's understanding of the situation, and signpost towards any major events or signijlcant changes in the weather. The amount of time given to this introduction should be varied

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depending on time of day, and day of week. Monday to Friday, For the 0603, 0655 and 0755 broadcasts, only the briefest preamble is needed, as most people are simply listening out for the mainlimportant elements. At 1255.1755. and more generally at the weekend, then some licence is possible; the listener is often more relaxed, and receptive to a rather more ‘rambling‘ style of preamble, though even here care is needed not to confuse, or worse, lose the listener, and thus create irritation.

(b) The main body of the broadcast. which should account for some YO per cent, should be a clear area-by-area weather fore- cast concentrating on the elements most of interest to the general public - stick to the facts i .e. rain/no rain, sunny/cloudy, windy or not. frosty or not, then a definite mention of temperutures and wind regime expected - distinct from the foregoing weather elements. I t is most important not to merge day and night. The human mind can and does divide the 24 hours into daylight and night-time. The forecast should always. without exception. be . . . ‘today’ then ‘tonight’, or *tonight‘ then ‘tomorrow’, with a clearly defined break between the two. Similarly. listeners expect the temperature and wind elements to come at the end o f the forecast for that area - if these figures are embedded within the forecast, the mind stops, registers the values, then misses the perhaps important element that the pre- senter was trying t o ‘wrap around’ the stated element. You can do this on television. because the viewer has a ‘back-up’ o f the displayed chart to refer to - not so on radio.

(c) A brief outlook, mentioning major changes only. unless more time can be wrested from the BBC for an outlook - something that ought really to be attempted, at least on Friday mornings (for the weekend). and also Saturday lunchtime, to allow more detail for Sunday (and Monday when a Bank Holiday). Be careful with this outlook though. I f you have ‘Hagged‘ a major change say several days ahead (e.g. warm now but turning distinctly colder by the end of the week), you are going to con- fuse if the formal outlook for the next day or so appears to contradict this - again. on television it is possible. though I’ve seen confusing charts there; on radio. if in doubt. leave it out! Stick to the short-term forecast.

Some other ‘Golden Rules’: ( i ) Neoer draw invisible lines dividing

up the country - only a very small proportion of people can visualise such splits. and whilst they are trying to decide whether they live east. west, north or south of this divide. the

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main body of the forecast has been and gone - you have wasted your time!

(ii) Unless there are good public safety reasons (e .g . violent snowstorms, severe gales, etc.). start the forecast in the south or south-east and work north - i .e. concentrate on areas where the majority of people live, and incidentally where the biggest Radio 4 audience is. It has long been recognised that the listening public gets used to a set order for the national weather forecast - any vari- ation is at best irritating, and at worst rnis- leading.

(iii) Keep the description within each area logical, along the lines set out above. and always mention temperatures (with some qualification if it would make things clearer). and wind. Most people listening just want to know whether they need an umbrella, can wear lighter clothes. should take the bus or walk, etc.

(iv) Avoid verbal distractions. which detract from the listener’s retention of the broad thrust of the forecast. Here I include ‘percentage probability’ of events - they sound scientific, but I doubt if the general populace can really retain and usefully apply such information. They work well on tele- vision, where the contrast between areas can be visually demonstrated. but on radio they are an unnecessary diversion. ‘Equivalent temperatures’ - again. acceptable on tele- vision, where they can be interpreted on the same chart with forecast wind and tem- peratures overlaid. and thus explained. or on local radio. However, there is no time on national radio for this, so stick to the facts!

(v) Actual values for wind speed should be mentioned only when such events are significant. i .e. gusts sufficient to cause danger to land operations. Gusts t o 30 mph. as heard recently, are not significant.

Members o f the television team will no doubt not be pleased with this criticism but n o personal attack is intended. and indeed they do excellent work on the medium in which they are trained. However. as Richard Dimbleby once stated (when the fledgling BBC Television Service was gearing up after the Second World War). the techniques for television are totally different from those required for radio. The BBC does not expect its news-readers to present the 6 o’clock TV news on BBCl, and then attempt to do the same thing on radio half an hour later - neither should we! The radio medium needs a different approach. and this should be recognised in the re-establishment of a dedi- cated radio weather presentation team.

St. Albans UARTIN ROWLEY

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