New Way To Make Rain?

Salty sea spray can cleanse the air of pollution, making rain as it scrubs the sky clean, Israeli scientists reported Thursday.

Rather than all falling back into the sea, some salty drops of water go up into the atmosphere and help make raindrops -- which in turn drag particles of pollution down into the water, Daniel Rosenfeld and colleagues at the Hebrew University of Jerusalem found.

The findings, published in Friday's issue of the journal Science, may help scientists find better ways to make rain, said Rosenfeld, a cloud physicist.

People have tried to make rain for centuries, often using salt, but no one knew the process was happening naturally at sea.

"The idea that larger salt particles can seed clouds and enhance rainfall is not new but it was not combined with actual observation," Rosenfeld said in a telephone interview. "It was believed that most of the sea spray was falling back to the ocean before it reached the clouds."

His team used satellite data to look at clouds forming over South Asia, finding that "huge amounts of air pollution" spilled off the continent over the Indian Ocean.

This pollution is known to cause rain over land, and it contributes to huge, towering storm clouds over India and Bangladesh. "They grow as high as 4 miles high over land and precipitate acid rain," Rosenfeld said.

But he found that over the ocean the clouds did not need to get so large before making raindrops.

"To my surprise I saw the clouds there precipitate readily even when they are not much vertically developed, in polluted air," he said.

Usually a cloud has to build up enough drops before they are heavy enough to fall in the form of rain. Sea spray seems to accelerate this process, Rosenfeld said.

"The sea salt particles, which are about 10 times larger, form much larger drops that start to fall and collide with the smaller, polluted drops and collect them into raindrops that contains this pollution and it rains out," Rosenfeld said.