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Does cloud seeding work?

China takes credit for the storms now bringing a reprieve from severe drought, but is that claim valid?



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At long last, it snowed in northern China. The first snow of the year came to Hebei, the northern province surrounding Beijing. In the Chinese capital, it was the first real bout of precipitation since last October. The blizzard caused 12 area highways around Beijing to close.

Travel inconveniences aside, the blast of moisture was more than welcome in China, which is suffering through its worst drought in decades. Earlier this week, government officials announced that even the country's massive water projects, like the North-South Water Transfer Project, couldn't hope to deal with future shortages—the country would have to cut back its demand as well.

It should come as no surprise, then, that the Chinese government wanted to claim a little credit for the good weather fortune, and they have. Officials said their cloud-seeding program directly caused the snowstorm. Engineers blasted more than 400 cigarette-size sticks of silver iodide into the sky shortly before the storm, and a senior engineer told Reuters that it was "a procedure that made the snow a lot heavier."

Cloud seeding and other weather-modification schemes have been around for years. But how do they work? And can you really claim that cloud seeding caused a particular storm? We asked cloud-seeding expert Arlen Huggins, associate research scientist in the division of atmospheric sciences at the Desert Research Institute in Reno, Nev., to give us some answers.

[An edited transcript of the interview follows.]

Tell us a little about the basics of cloud seeding.
Clouds, whether in summer or winter, are not perfectly efficient at producing precipitation. There's some part of a storm that's much less than 100 percent efficient in turning clouds into precipitation. In winter, the problem is that there aren't sufficient ice crystals. If droplets fall in liquid form they generally evaporate. The idea is to add ice-forming particles.

Why silver iodide?
One thing people discovered early on is that the structure is very similar to that of ice. The lattice structure at the molecular level is very, very close. We think that's why ice wants to bond to it.

When did scientists get serious about experimenting with cloud seeding?
I think it was the day after they finished the experiments in the lab [chuckles]. The initial discoveries were in the 1940s, with substances like silver iodide. It was a short period before they started trying to affect larger systems, without much success.

Wasn't Kurt Vonnegut's brother one of the lead scientists in the 1940s?
Yeah, Bernard Vonnegut.

[Editor's note: Bernard Vonnegut, the older brother of the late novelist Kurt, uncovered silver iodide's weather-modifying properties as a researcher for General Electric in 1946. He later taught atmospheric science at the State University of New York at Albany before passing away in 1997.]

What are your experiences with cloud seeding?
My initial experiences were using cloud seeding to try to reduce hail fall. This was in northeast Colorado, where they don't get the massive hail people get in the Midwest, but they got it at a higher frequency. It turned into more of a research project into how hail and thunderstorms work. There was very little evidence that seeding was reducing hail, but we had some success later in increasing snowfall in the Sierra Nevadas, and more success in the mountains of Utah. We were able, in some instances, to monitor the chain of events, following the storms with aircraft. Now we're studying the actual snow for trace concentrations of the seeding material to see how effective the procedure was.

Plenty of scientists and organizations have tried to write off cloud seeding. Why does it keep coming back?
It keeps coming back because of the demand for water, especially the dire straits of the arid Southwest. It's always been seen as a cheap way to add additional water.

What are the best results you can hope for with cloud seeding?
It depends on where you are, and the reasoning behind it. If you're trying to increase rain or snowfall for the water supply, a 10 percent addition could do a lot. If you had a large basin like I work with, between 300,000 and 500,000 acre-feet*, a 10 percent increase would equal 30,000 to 50,000 more acre-feet of water. If you can do that, it's very economically sound.

Can you attribute any one storm to cloud seeding, as the Chinese government has done with this snowstorm?
Generally, you can't look at one individual storm. Would it have snowed anyway? You don't know. NCAR (the National Center for Atmospheric Research in Boulder, Colo.) is in Wyoming studying several seasons in comparison, where you seed one place and withhold another. Attributing one storm to seeding is very difficult unless the cloud system is incredibly simple, like fog that has no chance of precipitation. If you see snowfall then, that's pretty demonstrative evidence that you succeeded.

Will China's cloud-seeding efforts help to end its drought?
If they are in a drought, they wouldn't be able to draw enough from cloud seeding, just for the lack of clouds. You treat the storms you have, so cloud seeding certainly isn't going to bring you out of a drought. The best time to do cloud seeding is when you have normal levels, or higher-than-normal levels, of precipitation. Then you could save the extra water in a reservoir for when you are in a drought. It certainly won't bring you out of one.

Is cloud seeding unnatural?
Those concerns generally come from places where there's never been a cloud-seeding project before. The problem with saying it's unnatural is that as a human species, since we first set foot on the planet—or at least since we started burning fossil fuels—we've been modifying weather systems on a much larger scale than cloud-seeding projects. We actually get more questions about the potentially harmful effects of chemicals like silver iodide. As a pollutant, silver iodide is almost overshadowed by smokestacks spewing kilotons of pollution, or by auto exhaust.

*Correction (2/25/09): The unit "acre-feet" originally appeared as "acres."

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