Murky past of cloud seeding

On 15 August 1952, the people of Lynmouth in south-west England experienced a downpour like never before. The ensuing floods killed 34 people and left 420 more homeless. At the time rumours circulated that the flood could have been the result of cloud seeding trials that the Royal Air Force was conducting nearby. However, few people now think that the cloud seeding was to blame, because the RAF was seeding cumulus clouds, while the rain that deluged Lynmouth came from a large depression sitting over the region.

In 1966 the US military began operational flights on the CIA-inspired, top secret "Project Popeye". Their aim was to extend the monsoon season over south-east Asia, thereby increasing the amount of mud on the Ho Chi Minh Trail and flooding critical routes between what were then North Vietnam and South Vietnam. For seven years, aircraft flew more than 2600 sorties to disperse silver iodide into the clouds over the region, and initial results were positive, although analysts remain divided as to whether the extra mud on the trail really made much difference.

On 9 June 1972, 238 people lost their lives in floods in Rapid City, South Dakota, when nearly a year's worth of rain fell in just a few hours. Cloud seeding had been carried out nearby earlier in the day and many people believed this triggered the storm. But Harold Orville, who was involved with the seeding programme, is certain that it was not responsible. "Our seeding was done on the plains, 20 or 30 miles from the area where the heavy rains fell. Numerical simulations of the weather that day suggest that the flood would have happened anyway," he says.

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