China's Rain-Free Olympics Plan Met With Skepticism

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"They have about 30,000 people and some $100 million U.S. dollars a year invested in this field," Bruintjes said.

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.

(Read about China's Olympic boom in National Geographic magazine.)

Xinhua, China's state news agency, reports that years of such efforts have produced billions of tons of water.

More than 40 countries host cloud-seeding projects worldwide, targeted to benefit businesses from farms to ski resorts. But there is still no agreement on how well it works.

Quandary

A host of factors go into producing precipitation, but they create a simple quandary: It's difficult to tell whether a seeded cloud would have yielded rain or snow on its own.

Natural variability further confuses the issue because what works in one location, or during a certain season, may function entirely differently at another place and time.

Scientists use several methods to try to create rain or snow, but all need existing clouds to work.

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.

(See a picture of silver iodide being dropped over Kansas croplands.)

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.

"Under the right conditions we can increase snowfall," said Arlen Huggins, a Desert Research Institute scientist and director of the Nevada State Weather Modification Program.

"In some places we've seen increases of 10 to 15 percent."

Pollution Impact

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.

(Related: "Asia Pollution Changing World's Weather, Scientists Say" [March 6, 2007].)

Constant seeding by pollution particles may actually decrease regional precipitation.

In North America, the effect appears strongest in mountain ranges such as the Sierra Nevadas, which lie downwind from large, smoggy cities.

Normal snowpack appears to have decreased in such regions, Golden noted.

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