Good Day Sunshine was never meant to be a statement of fact, but this weekend, Macca meant it. When his 3,000th concert in St Petersburg's Palace Square was threatened by poor weather, his concert promoters promised that the sun would be shining by the time McCartney took to the stage.

A Moscow company, SAV Entertainments, arranged for three jets to streak over the pregnant skies, dumping vast quantities of dry ice, or frozen carbon dioxide, as they went. By doing so, they hoped to "lock" the rain into the clouds, ensuring a dry spell over the city.

According to McCartney's spokesman, Geoff Baker, who was at the concert, the £20,000 plan worked a treat. "It was pissing down beforehand and then it became so sunny that Brian Ray, the guitarist, had to get one of his roadies to fetch his shades," he says.

The idea of cloud seeding, as the technique is known, is simple and dates back to the 1940s when some believed that to control the weather would be to control the world. In theory, dumping dry ice into clouds can do one of two things. Use a little, and water droplets in the cloud will freeze, grow and then fall as ice particles, lessening the chance of rain later on. Use a lot, however, and the ice particles that form will be so tiny and light, they won't fall, effectively locking them into the cloud. That's the theory, at least.

In practice, scientists are sceptical that such simple measures could take charge of the weather. One problem that has dogged research into cloud seeding for decades is that it's so difficult to prove. "You can't know what would have happened, whether it would have rained or not, if you hadn't done anything," says a Met Office spokesman.

Anthony Illingworth, a meteorologist at Reading University, agrees: "Some people have made a lot of money out of it, but because it's so hard to prove it's worked, we tend to view it with considerable jaundice."