How does cloud seeding work?

Answers to your questions about the news.
May 11 2005 6:46 PM

Can the Russians Control the Weather?

How cloud seeding works.

Always take the weather with you 
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Always take the weather with you

Last weekend, Russian president Vladimir Putin ordered the Russian Air Force to keep it from raining over Moscow in advance of Monday's military parade. The Air Force used a procedure called cloud seeding, and Russia's defense minister later took credit for Monday's sunshine. Can the Russians really control the weather?

Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

They can try. Proponents of "cloud seeding" say it's possible to induce rain and snow (after which clouds can break up and disappear), suppress hail, and clear up patches of fog. Twenty or 30 countries run cloud-seeding operations of some sort; China has spent hundreds of millions of dollars on weather manipulation over the last decade. The bureau in charge of cloud seeding in Thailand reportedly has 600 staff-members and a $25 million budget. No federal funds go toward cloud seeding in America, but a handful of states finance projects locally. Utah just kicked in for $400,000 worth of weather control projects.

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Here's how it works: Rain starts as tiny droplets of water suspended in clouds. Then the droplets clump together into bigger drops (or freeze together into bigger crystals). Once the drops or crystals are big and heavy enough, they fall out of the sky. The frozen drops can melt on the way down, becoming rain, or they can fall to the ground as snow. Cloud seeding aims to jump-start this process by helping droplets to clump or freeze together when they otherwise wouldn't.

Cloud seeders use a number of procedures. In the mid-1940s, three scientists at General Electric (including the novelist Kurt Vonnegut's brother Bernie) showed that by injecting dry ice into a cloud, you could freeze tiny droplets of water, which would in turn make it easier for other droplets to glom on and freeze as well. Later experiments showed that silver iodide—which has a crystal structure similar to that of ice—could also help, by forming "ice nuclei" upon which droplets might freeze.

To induce rain with dry ice, you would fly a plane over a small cloud and sprinkle down a few cups' worth of dry ice pellets. To seed with silver iodide, you'd vaporize a solution at high temperatures and disperse it in the cloud. This can be done using silver iodide flares, which are dropped 8 or 10 at a time from above the cloud, or with silver-iodide-filled rockets or anti-aircraft shells. If you're seeding clouds over a mountain, you can use generators on the ground which release silver iodide vapor into the air currents that rise up one side of the mountain and into the clouds.

Silver iodide and dry ice are examples of "glaciogenic" agents, and they only help to produce rain in clouds of sufficiently low temperatures. For warmer clouds—in which droplets don't freeze before falling—cloud seeders can use sprays of saline solution to attract droplets and, theoretically, to induce rain.

It's difficult to prove whether cloud seeding actually has any effect. Weather phenomena are so variable that slight changes in the probability of rain are difficult to measure, and not many careful, controlled studies have been done. The U.S. federal government was at one time very optimistic about weather manipulation; by the late 1970s, annual funding for cloud-seeding projects hit $20 million. But after years without definitive results, interest in Washington has evaporated (except, perhaps, among the people who introduced this bill in the Senate in early March). Some studies have suggested that cloud seeding actually reduces rainfall, or merely redistributes it. A 2003 report from the National Research Council concluded that while cloud seeding may hold promise, we still don't know very much about it.

Explainer thanks Don Griffith of North American Weather Consultants and John Marwitz of the University of Wyoming.

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