By Crispian Balmer
With forecasters predicting a 41 percent chance of rain on the day of the August 8 opening ceremony, officials have said China was considering deploying experimental technology to try to ensure dry weather and clean air for Friday.
However, many other nations have abandoned such meteorological manipulation projects and state research has largely dried up because of nebulous results.
"The most recent scientific assessment says there is very, very slim evidence of the success of weather intervention in increasing, decreasing or preventing rain," said Leonard Barrie, co-director of the World Meteorological Organization research department, based in Switzerland.
"In a way you can regard Beijing as a demonstration of their weather modification capabilities," he told Reuters.
August is the rainy season in Beijing, with thunderstorms a regular feature that could wreak havoc with the Games.
Organizers are especially anxious about the lavish opening ceremony which will be played out in the national "Bird's Nest" stadium in front of a TV audience to be numbered in the billions.
"We will see if certain weather conditions will affect Beijing and if we need to apply certain techniques," said Zhang Qiang, deputy head of the Beijing Weather Modification Office.
There are two main methods of controlling rainfall.
Either the Chinese could try to induce rain before it reaches the city centre, by firing a chemical agent into clouds to "seed" them and make them more efficient at generating ice crystals that melt to produce raindrops -- effectively draining the skies.
Otherwise they could use a coolant that increases the number of water droplets in the cloud formations, thereby decreasing their size and making them less likely to fall as rain.
Cloud-busting technology has already been deployed at previous sporting events, and with considerable success if one believes the assertion of Russian scientists.
They have said their "cloud-seeding" activities kept the rain at bay during the 1980 Moscow Olympics and again during the fortnight of the Goodwill Games in 1994 in St. Petersburg.
But many scientists are skeptical about the real success of manipulation programs, saying it is very hard to assess how much rain any seeding really provokes.
"Although the principles behind it are well established, it is difficult to prove that a given round of cloud seeding produced a particular effect," says the U.S. National Center for Atmospheric Research based in Colorado.
Two years ago, China was bullish about its capabilities, saying 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.
But the Chinese Weather Modification Office has itself sought to play down its chances of success should bad weather threaten to rain on its athletes' parade, suggesting it is powerless to halt the progress of thick storm clouds laden with water.
"(Modification) of clouds and rain are only at the early stages of experimentation," deputy director Zhang said.
(Additional reporting by Lucy Hornby)
(Editing by Ed Osmond)