Wednesday, May 2, 2007

China’s plan to keep the skies sunny and clear during the 2008 Summer Olympics in Beijing shows how far weather modification has come, climatologists say.

If a storm approaches the city, the Chinese said they would seed the clouds with silver iodide to force rainfall, cleansing the air and ensuring spectators and athletes stay dry.

Weather patterns over Beijing in recent decades indicate a 50 percent chance of rain, Chinese meteorologists say. Air pollution from industry and automobiles also is a problem.

The Chinese are as likely as anyone to have scientists skilled enough to stop the rain, said Joseph Golden, senior research scientist for the Cooperative Institute for Research in Environmental Sciences in Boulder, Colo.

The Chinese government has one of the world’s largest weather-modification programs, spending about $100 million on the projects and training about 1,500 scientists to administer them, he said. Most of the effort involves increasing rainfall for agriculture.

Weather modification in the United States is more sporadic and done mostly by state or local governments.

“We have fallen behind,” Mr. Golden said.

The techniques have been used to increase snowfall in the Sierra Nevada Mountains of California, to clear up icy fog around Salt Lake City International Airport, decrease the size of hail in North Dakota and increase rainfall in Texas.

Changing the weather for a few days during the Olympics is a step beyond any previous efforts by the Chinese, he said.

“This is a whole new thing for them,” Mr. Golden said.

Cloud seeding is nothing new, but doing it successfully to prevent rainfall in a specific place is a tricky proposition, according to climatologists.

“I think there’s a good chance” Chinese scientists can prevent rain from falling on the Olympics, said Ross Hoffman, vice president of Atmospheric and Environmental Research Inc., a Lexington, Mass., company that does customized weather forecasting for industry.

When silver iodide is dispersed in clouds, it can cause moisture to condense, similar to the way water condenses on the outside of a cold drink in a glass on a hot day, Mr. Hoffman said. However, if too much silver iodide is added to clouds heavy with moisture, it can overwhelm their ability to condense, thereby stopping the rain.

Scientists must make educated guesses about how much silver iodide should be dispersed and the best time for it.

“It depends on the cloud,” Mr. Hoffman said.

They also must choose the appropriate method.

For much of the half-century history of cloud seeding, generators on the ground have been used to create a mist of silver iodide particles that float upward toward the clouds.

Pyrotechnic rockets also have been used to shoot tablets of the chemical into the sky, where it explodes and spreads into the moisture.

However, rockets are only as good as the person who aims them and often disperse the chemical too randomly to be effective, climatologists said.

A more modern method is to disperse the silver iodide through flares on airplanes. As the flares burn, the chemical particles disperse at a uniform rate in clouds. They can either be dropped from airplanes or attached to the wings.

Regardless of the scientists’ skill, Mother Nature must cooperate for weather modification to succeed.

“You can’t make rain out of a clear sky,” said Doug Lilly, a retired University of Oklahoma professor of meteorology.

In addition, tampering with the weather can create unplanned consequences, he said.

“There is also the chance that the seeding will produce hail where it wouldn’t have happened otherwise,” Mr. Lilly said. “I wouldn’t want to be subject to the lawsuits.”

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