Beijing plans to make rain to ensure a fine Olympics

Ravaged by a seven-year drought and choked by poisonous dust storms, China will "seed" the clouds looming over its cities with rockets and chemical pellets to bring rain and so clear its polluted skies for the Olympics in 2008.

China leads the world in using cloud seeding to induce rainfall. In the past five years, the practice has produced enough precipitation to fill the Yellow river, the country's second longest, four times over.

It has become a tradition in Beijing to seed the clouds before public holidays to make rain, disperse pollution and ensureclear skies.

China has pitched the Beijing Olympics in 2008 as the "Green Games" and will be viewed around the world as a showcase for the country's efforts to combat pollution and encourage sustainable energy use. The World Bank says 16 of the world's 20 most polluted cities are in China, including Beijing. The Games will be the biggest test to date of cloud seeding.

There is an ongoing debate among meteorologists about the effectiveness of the practice, though most agree the principle works. Some studies show cloud seeding can increase precipitation by up to 30 per cent, but others are wary of the overall effect on the environment.

In the past five years, Chinese air force jets have flown nearly 3,000 flights and sowed rain-making chemicals to bring down 210 billion cubic metres of water over three million square kilometres, nearly a third of China's territory.Rain-making rockets and shells were used in 1,952 counties across the country, and 3,000 people are employed in the business, with a formidable arsenal of 7,000 cannon and 4,687 rocket launchers.

Cloud seeding is becoming increasingly common in China as a means of offsetting the effects of drought. Much of the country is short of water, and the water table is dropping quickly in many places. Beijing residents were warned this week to start taking action to save water or face a massive shortage of water by the time of the Olympics in 2008.

Weather specialists induced rain in early May in Beijing to help relieve drought and wash tons of dust from the Gobi desert dumped on the capital by a rash of sandstorms. Cloud seeding is also used to prevent hail and extinguish forest fires. Artificially induced rain was used to help put out three major forest fires that raged in north and north-east China for 10 days before they were subdued last Friday.

Cloud seeding began in the 1940s. The process involves injecting particles into a cloud, which act as freezing nuclei. Cloud droplets stick to the injected particles and fall to the ground as rain or snow. The right clouds are needed for cloud seeding to work.

The earliest attempts at cloud seeding involved dropping pellets of crushed dry ice, or carbon dioxide, into the top of a cloud. Later, scientists started to use silver iodide because it was more effective than dry ice.