A worker checks a cloud seeding cannon near Fragrant Hills in Haidian district Wednesday to prepare for a third potential snowfall. Photo: IC

By Zhang Hui

The cloud-seeding chemicals used to prompt Beijing's recent snowfall will not affect either the environment or humans, meteorologists assured Wednesday.

Although this winter's third forecasted snowfall left the city hanging Wednesday, online forums saw a blizzard of discussion regarding the cloud-seeding chemicals used to coax precipitation from the clouds in this year's two preceding snows.

Most Web users were worried that the chemicals involved might include heavy metals that could harm the city's environment and its residents.

"Dear friends, stop playing in the snow! It contains a great deal of silver iodide, which might affect our health," a Web user named Xiao Ke said on a microblog at sina.com.cn.

But research by international meteorologists has showed that silver iodide, the snow-making catalyst, has no harmful impact on the environment, an officer surnamed Fang of the Weather Modification Center of the China Meteorological Administration told the Global Times.

"We only use a very small amount of silver iodide scattered over a very large area," Fang said.

About 13 kilograms of silver iodide were dispersed in the clouds over the city's nine districts and counties, or 10,000 square kilometers in total, for the two snows after Spring Festival. That means every square kilometer of earth received only 1.3 grams of silver iodide, Beijing Weather Modification Office director Zhang Qiang told the Beijing Morning Post Wednesday.

"The snow that falls from cloud seeding does not contain any polluting chemicals. It's the same as snow that falls without a catalyst," Fang said. "It won't affect human health."

"The cloud seeding cannons are in the suburbs, and we only use them to increase snowfall in these areas in hopes of relieving the drought," a municipal weather office official told the Global Times.

In the process, silver iodide is sprayed into the sky, causing water in the air to condense and form clouds that ideally will produce snow, though it only works if water is already present. "We can make more snow only when snow already exists," Sun Jisong, chief meteorologist at the Beijing Meteorological Bureau told the Global Times Wednesday.

Cloud seeding can only generate a very limited amount of precipitation for a very small area, Sun added.

"The meteorological department seized the right time for making snow, and snow increased by 20 percent in the affected areas during the recent two snowfalls," Zhang was quoted as saying. She explained that snowflakes are notably larger and that snow density increases when a silver iodide catalyst is used.

"The weather will turn warm gradually, and there are few chances for snow again this winter," Sun said.