"The Day They Made It Rain" was the provocative and inaccurate title of a recent Radio 4 programme. Its aim was to show that cloud-seeding experiments may have been responsible for the disastrous Lynmouth flood in 1952. That flood destroyed much of that north Devon village on the night of August 15-16, leaving 34 people dead and 420 homeless.
The radio programme presented evidence of rain-making experiments in southern England in the early-1950s thanks to the detailed recollections of RAF personnel, now long retired. These airmen remembered meteorologists supervising the seeding of selected clouds, and the great satisfaction of the scientists when rain was subsequently reported. It was also claimed that these exercises were conducted in secret.
All this falls into the "Never let the facts get in the way of a good story" category. Far from being secret, rain-making experiments were talked about all over the place in the early-1950s. The Royal Meteorological Society's popular magazine, "Weather", devoted a whole issue to the subject in July 1952 - just a month before the Lynmouth disaster. One article by E.G.Bowen described a systematic research programme of artificial stimulation of rainfall in Australia which had begun in 1947, while one of Brita in's foremost atmospheric physicists of the day, Frank Ludlam of Imperial College, described in detail the physical processes underpinning cloud-seeding research in the UK.
Any meteorologist with a rudimentary knowledge of cloud seeding could explain why it is preposterous to blame the Lynmouth flood on such experiments. The key to understanding why this is the case is that completely different rain-making processes are involved. Scientists involved in rainfall stimulation were only interested in seeding individual cumulus clouds - those cauliflower-shaped clouds, usually less than a mile across, which sometimes produce showers which may last 10 or 20 minutes. Injecting modes t amounts of dry ice or silver iodide into such a cloud stimulates the production of ice-crystals in the cloud which in turn accelerates the rain-making process. But the cloud has to have sufficiently vigorous updrafts to spread the chemical throughout the cloud. This in turn means that the cloud may have eventually produced rain in any case, and the seeding merely caused it to happen earlier. For this reason there has never been unequivocal evidence of how successful these rain-making programmes have been.
The storm which caused the 1952 disaster was not confined to the Lynmouth district. Heavy rain fell over the whole of the West Country and south Wales, and it was caused by a depression which had stagnated in the Southwest Approaches for two days. Similar depressions have triggered serious flooding in southwest England at regular intervals, and previous devastating floods hit Lynmouth in the 18th and 19th centuries. The August 1952 depression was several hundred miles across, and the prolonged heavy rain a ssociated with it was caused by the large-scale lifting of very moist air. A fleet of RAF Hercules stuffed with dry ice wouldn't have made a ha'p'orth of the difference.
