Clarification: This article did not clearly define the toxicity of silver iodide used in cloud seeding. The chemical is listed by the Environmental Protection Agency as having a low level of toxicity. In cloud seeding it is applied in amounts so small they have not shown toxic effects in soil, water or vegetation, according to the California Water Plan.
SOUTH LAKE TAHOE, Calif. - Even weather modification experts are having a difficult time manipulating much white stuff from this year's clear skies.
"You can't just make it snow or rain," said Hans Ahlness, vice president of operations for Weather Modification Inc., which operates cloud seeding projects in the American River watershed. "You have to have the cloud there and all you can do is make it more efficient."
Cloud seeding is a method of enhancing precipitation that falls from storms. Silver iodide, a chemical similar in molecular structure to ice particles, is either dropped by an airplane or released by a "generator" upwind of storms. Dry ice is sometimes also used in airplane seeding. The tiny particles give moisture in the clouds something to condense around and cause it to fall, as rain or snow, from the sky.
The Sierra Nevada has been under cloud seeding projects for decades. Water managers in California and Nevada have contracted the service to augment the area's snowpack and improve supply.
"We're just trying to add a little more water to the runoff," said Arlen Huggins, an associate research scientist with Desert Research Institute, which does cloud seeding in the Tahoe-Truckee watershed.
Cloud seeding in California can add as much 300,000 acre feet or about one four-hundredths of Lake Tahoe to the state's annual water supply. Depending on the storm, cloud seeding can add 5 percent to 10 percent of precipitation than would've dropped normally, said Maurey Roos, a hydrologist with the Department of Water Resources.
But conditions must be right for seeding to be effective. Air temperature at storm altitudes, usually around 9,000 feet, must be around 23 degrees Fahrenheit. The storm must contain "super-cooled" liquid, or moisture that's colder than freezing, and must not have too strong of winds, Roos said. And for airplane seeding, the storm must be high enough off the ground for the craft to fly into.
So far this year hasn't produced too many chances. In the Tahoe-Truckee area, there have only been six cloud seeding operations from their ground generators so far. Last year, Desert Research Institute conducted 25 to 30 by March, Huggins said. In the American River area, the airplanes have seeded seven times, Roos said.
"They haven't had a lot of opportunities," Roos said. "You're not going to create clouds or snow out of a clear sky."
Needless to say, water managers aren't looking at weather modification as a solution to the abnormally dry winter, though it may help a little bit.
"The amount you add from cloud seeding is not going to change the overall conditions of a dry year," Huggins said. "Like right now, we're running about 30 percent water content of normal. If we add 10 percent, it's not going to make much of a difference."