In California, seeding clouds to make it rain could be a matter of life or death. Amid an emergency state-wide drought, officials in Los Angeles have resumed a controversial programme developed in the 1940s to fire chemicals into the sky.

The £400,000 cloud-seeding project will involve injecting clouds above the San Gabriel Mountains with silver iodide particles in an effort to increase rainfall by 15 per cent.

“We are helping Mother Nature along a bit,” said Don Griffith, the president of North American Weather Consultants, which has held various cloud-seeding contracts with Los Angeles County since 1957.

Critics have argued that conservation is a more effective strategy and that the rainmaking experiments will take place too close to residential areas, leaving them vulnerable to flooding and mudslides.

The chemistry involved in cloud seeding was discovered in 1946 by a meteorologist named Vincent Schaefer, who worked at the research division of General Electric. It was refined by Bernard Vonnegut, the brother of the late author and Second World War veteran Kurt Vonnegut, and was used in an experiment in the 1960s known as Project Stormfury, which attempted to modify the course and severity of hurricanes.

A report published by the US National Academy of Sciences in 2003 said that the science remained unproven despite more than 30 years of trying. It said: “There is still no convincing scientific proof of the efficacy of intentional weather modification efforts.”

It is believed that rainmaking experiments by the RAF in 1952 caused 90 million tonnes of water to fall into the Lynmouth valley in North Devon, killing 35 people. Survivors said that the air smelt of sulphur and that the rain fell so hard that it hurt people's faces. Any knowledge of such weather experiments has been denied by the Ministry of Defence.

Even California has bad memories of cloud seeding. In 1978 a storm caused devastating floods in a part of the San Gabriel Mountains called Big Tunjunga Canyon, killing 11 people and causing $43 million of damage. Many residents sued Los Angeles County, claiming that cloud seeding had worsened the rainfall. The county prevailed in court but stopped all rainmaking experiments until 1991. They were stopped again in 2002 when it was feared that wildfires had left areas vulnerable to rain-induced mudslides.

Because the water system in California - and therefore its economy - is so dependent on rainfall, interest in cloud seeding has always heightened during droughts. It is thought that until recently the Los Angeles Department of Water and Power seeded clouds in the Eastern Sierra Nevada Mountains, which provide much of the water supply for Los Angeles.

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