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Extreme summer storms of the sort that hit Beijing on Wednesday have become more frequent over the past decade and work is continuing on finding ways to protect Olympic events from them, officials said on Wednesday.

Workers take shelter in pipes at the construction site of National Olympic Stadium, also known as the Bird's Nest, as a rainstorm hits Beijing June 27, 2007.[Reuters]

"The average temperature has been increasing, which is in line with international trends," Wang Jianjie, deputy director of the Beijing Meteorological Bureau, told a news conference.

"Since the 1990s, there has been an increase in very high temperatures in the summer (and) annual precipitation has become lower but localised extreme rainfall is increasing."

Beijing is particularly concerned about a storm at the opening ceremony of next August's Olympics after organisers decided to dispense with a roof over the main "Bird's Nest" National Stadium as a cost-cutting measure.

Zhang Qiang, head of the office of weather manipulation, said in April that the bureau would be able to offer a guarantee of dry weather immediately around the stadium after tests of "artificial mitigation of rainfall".

Her colleague Wang Yubin said on Wednesday that while the science of enhancing rainfall by "seeding" clouds was relatively advanced, the art of dispersing them to prevent precipitation was still in its infancy.

"Dispersal is more difficult than rainfall enhancement, it is less mature," he said. "We have to have very good timing and be very accurate.

"We have to continually seed the clouds to modify them so the stoppage takes place over our target area.

"So the forecasts will have to be very accurate, which makes our job even harder.

"The kind of chemicals we use to seed the clouds also makes a difference to the effectiveness. We are still in the experimental stage but we are continuing with our efforts."

Beijing, which is chronically short of water, frequently benefits from the bureau's work in enhancing rainfall.

While the downpours also have the effect of clearing the city's notoriously polluted skies for a few days, the use of chemicals brings with it environmental concerns.

"We have been inspecting the water of the reservoirs within our working area since 2004," said Zhang Qiang, adding the chemical silver iodide was only used in small doses and did not pose an environmental concern.