The Air Force of China's People's Liberation Army (PLA) has conducted the largest ever weather modification to ensure a clear sky above the Tian'anmen Square where the ground formations and aircraft will pass through during the grand National Day parade on Thursday morning.

To deal with possible rainfall or thick cloud that would impair or shadow the once-for-a-decade military parade, the PLA Air Force has deployed 18 aircraft which could spray environmental-friendly activators to seed rain-bearing clouds.

A shower washed Beijing Wednesday night and early Thursday morning after rain-inducing chemicals were fired into the sky above Beijing that was filled by smoke and vapor in the past two days.

Another 48 fog-dispersal vehicles have also been on stand-by beside runways of the airbases from which all 151 aircraft performing the air display could have a safe take-off and landing.

Beijing's civilian meteorological organizations also supplement the military effort with rockets fired into the air in the surrounding provinces and regions to load clouds with silver iodide or dry ice to induce precipitation before the rain reaches the center of the city.

The Air Force has dispatched more than 260 technicians and experts to help the ground crew in the airbases operate those weather-intervening equipment.

According to sources, the Air Force has been studying on the artificial measures to intervene natural weather phenomena since the 1950s. This is the first the largest number of personnel and equipment were involved for weather intervention for National Day parade.

Cui Lianqing, an senior air force meteorologist, said it has been the first time in the country's history to attempt such a large scale weather modification which could dwarf the rainfall stimulation operation for the opening ceremony of the Beijing Olympic Games last summer, when thousands of rain-dispersal rockets were launched.

If the rain-bearing clouds appear in the sky above the parade area, the aircraft will spray rain-stimulating chemicals; and if the rain-bearing clouds form just above the area, they will spray excessive burning silver iodide to withhold the raindrops for a certain period of time, Cui said.

"Only a handful countries in the world could organize such large-scale magic-like weather modification," said Cui. "However, the technology we have mastered so far could only allow us to modify the weather to a limited extent.

"There are many uncertainties up in the sky," he added.