Suddenly, on Friday evening, a rainstorm emerged, apparently from nowhere. In fact, it was "rocket rain".

For its true genesis lay in shells fired by technicians of the Beijing Weather Modification Office. They contained cigarette-sized sticks of silver iodide, which produce ice crystals when seeded within moist clouds, and turn to rain as they fall to warmer air.

The skies then cleared, producing a miraculously fine Sunday. The city's 15 million or so inhabitants, enduring the worst drought in northern China for 50 years, applauded.

But a water technology expert said yesterday that the impact of a single downpour on Beijing's steadily dwindling water table would only be marginal, because of the extent to which the city's surfaces are compacted by building works and new roads. The result is that most of the rainwater evaporates before it can find open, receptive ground into which to sink.

China and Thailand are the major users of artificial rain. In China's new five-year plan, it says it will create 50 billion tonnes of rain every year, deploying greatly increased resources into rain-making research, which it began in 1958.

The capital, where rainfall is down 63 per cent this year, is now expected to experience increasing numbers of such rain events as the Olympics approach.

The Government remains determined that despite the increasing frequency of sandstorms - with 300,000 tonnes of sand dumped in one morning last month - the 2008 Games will be the "Green Olympics". August, when the Olympics are scheduled, is also an especially unusual time for sandstorms in the capital. Beijing's Olympic planners are aiming for 292 "blue sky" days a year by 2008.

The longer-term fallout from seeding rain remains unknown.

People in rural Hebei province, which surrounds Beijing, have complained through local media that producing rocket rain for the capital diminishes their own rainfall, which has fallen 68per cent this year.

Clouds are required for seeding to be successful and forcing the clouds to produce rain in Beijing, Hebei inhabitants claim, leaves the surrounding countryside bereft.

Throughout the country, 23 of the 34 provinces have established weather modification bureaus like Beijing's, which use dozens of aircraft and several thousand anti-aircraft guns, as well as about 4000 rocket launchers, to seed clouds with iodine to produce rain.