"We have worked with (Beijing organisers) BOCOG on a contingency plan for a storm and other weather risks during the ceremonies which also involves neighbouring provinces," said Wang Yubin, deputy chief of the Olympic Meteorological Service.

There is a 47 percent probability of rain during the opening ceremony although the risk of heavy rain is only 6 percent. The Games close on Aug. 24 with another ceremony.

Strong winds and fog would also be troublesome because the "Bird's Nest" National Stadium, where the ceremony will be staged, has no roof.

"We hope we are able to solve these problems through liaison with neighbouring areas," said Zhang Qiang, head of Beijing's Weather Modification Office.

Seeding the clouds with chemicals fired from the ground or released from aircraft would trigger rain before it reached Beijing.

"Studies have proved that the chemicals we use do no harm to the air, water or the entire environment," said Zhang.

Trials aimed at reducing rainfall will continue in June and July in a project that has benefited from 3.5 million yuan ($500,600) of government support in the past five years.

However, it has proved impossible to stop rain storms.

"If it really rains heavily these methods would not be that effective," she said.

"I hope God will not send a storm to Beijing."