However, lack of proof that it works means that funding by many governments has fallen sharply, after millions of dollars were spent on teasing rain from clouds in arid regions of West Africa, or on research into trying to prevent hurricanes.

"There used to be big optimism about weather modification in the 1960s and 1970s," said Slobodan Nickovic, the World Meteorological Organization's expert on changing the weather.

"But so far fog dispersal...is the only activity where we have a high level of confidence in the technology," he told Reuters. Dispersing fog is useful, especially around airports, but hardly a solution to humanity's wider woes with the weather.

A 2005 WMO report expressed confidence that human use of chemicals could affect cloud formation but said there was only "medium" or "low" confidence that the changes lead to significant changes in rains, hail or snow on the ground.

It said the "unsatisfactory status" of weather research reflected a lack of understanding of the complexities of clouds. Rain dancers can sometimes claim credit for a downpour, even when clouds were about to burst.

"Most questions about weather modification are still open," Nickovic said, adding that more research was needed.

NO RAIN ON OLYMPICS PARADE

Some countries disagree.

Russia claimed credit last year for drying up rains that had threatened a Moscow parade celebrating the end of World War Two, attended by President George W. Bush and other world leaders.

Eleven planes seeded clouds with chemical dispersal agents under techniques Moscow says it has perfected over decades and that it says were used to keep the 1980 Moscow Olympics sunny.

China said in early June that it had created the "world's leading force" in making rain.

"Its aircraft alone have undertaken enough missions to fill four Yellow Rivers, the country's second longest river, in the past five years," the official Xinhua news agency said, citing the National Meteorological Bureau.

China also hopes to keep the weather under control for the 2008 Olympics. The weather scheme employs more than 3,000 people with 7,000 cannon and 4,687 rocket launchers.

China's "claims...are wildly optimistic, unreasonable and unproven", said Daniel Breed, a scientist at the US National Center for Atmospheric Research who works on a cloud seeding project in Wyoming.

He said China had some good scientists but that "they have not produced any -- or very little -- credible evidence". He said that there was evidence from countries such as the United States, Mexico and South Africa that it was possible to affect rainfall to help irrigation.

Under cloud seeding, chemicals such as silver iodide are released by rockets or aircraft, creating tiny particles to attract moisture and eventually get big enough to fall as rain.

There are limits -- clouds can be helped to form faster but cannot be conjured from a dry blue sky, Breed said.

Some environmentalists worry that repeated use of chemicals could damage crops. Others object that any rains coaxed from the sky in one region just mean less downwind.

"Except for China, funding has gone down to almost zero in many countries," said Douglas Pattie, an environmental officer at the secretariat of the UN Convention to Combat Desertification.

In 2003, the US Academy of Sciences said more research was needed, noting that US investments in weather modification had sunk to less than US$500,000 a year from US$20 million in the 1970s.

In one scheme, Washington tried and failed in the 1960s and 1970s to weaken hurricanes with chemicals dropped by planes. More recent suggestions, dismissed as unworkable, have included towing icebergs to the Caribbean to cool storm formation.