The Chinese want nothing left to chance at this summer's Olympics—not even the weather.

State scientists hope to provide blue skies for the opening ceremonies by "seeding" clouds to squeeze rain from them in the days before the games begin...

Human efforts to manipulate the weather have a long history.

Scientists have been trying to seed clouds and produce rain since the mid-1940s. During the Vietnam War, the U.S. military even tried to swamp the Ho Chi Minh Trail by producing torrential rains.

Military use of weather modification has since been banned by an international treaty since 1978.

China's ambitious Olympic plan is just a small part its weather modification program, the world's largest.

An array of rockets and anti-aircraft guns are used to "seed" clouds with particles to induce rainfall for the country's parched northern region...

The most common technique is to fill clouds with particles such as silver iodide that resemble the ice nuclei on which water accumulates to form precipitation.

The larger numbers of nuclei encourage more efficient growth of the larger crystals that will eventually fall as snowflakes or, if temperatures are higher, rain.

Another seeding technique employs particles such as potassium or sodium chloride, which attract salty water to build bigger raindrops.

The supersize droplets fall from the cloud more quickly and stimulate rain by colliding with existing smaller drops.

Some studies suggest that these methods can significantly boost precipitation...

Yet even with such successes, scientists stress that most human weather modification happens through pollution sent into the atmosphere each day.

"If you have driven your car this morning, you have been cloud seeding because of the particles emitted by your car," Bruintjes said.

Joe Golden is a senior research scientist at CIRES and the NOAA Earth System Research Laboratory.

"[The pollution effect] concerns us greatly," Golden said.

"In addition to whatever role greenhouse gases may play in global warming, it turns out that aerosol [particles] in the atmosphere that accompany pollution from coal-fired power plants and vehicles can impact rainfall and snowfall," he said.