Not all storms are efficient at producing precipitation that will fall to the ground. Cloud seeding aims to increase the number of ice particles in the right kind of storm, and more ice crystals means more snow.
According to Scientific American, Silver Iodide is the most well known substance that, if blasted into the right kind of storm, will cause more ice crystals to form in ways that will bring more snow.
Dry ice is another substance that causes ice to bond and build bigger crystals. According to Thredbo, the term is “artificial ice nucleating particles” and the process is called “ice nucleation”. The seeding agents are chosen for molecular structures that cause more ice crystals to bond to the agent. This makes more ice which becomes more precipitation.
The different seeding agents include dry ice pellets, some bacteria, liquid propane and silver iodide. The decision to use one agent or another depends on the local conditions.
The concept of cloud seeding goes back to the 1940s, when the idea was discovered, but there was no success in getting more ice. Author Kurt Vonnegut’s brother, Bernard Vonnegut, was one of the lead scientists who uncovered silver iodide’s weather-modifying properties as a researcher for General Electric in 1946.
Thredbo says that cloud seeding has been a success for the past 50 years in the U.S. and the past 40 years in New Zealand. A reasonable goal is to seed clouds over a large basin or to enhance the snow pack for a large watershed. If any increased precipitation comes from seeding, it will add more water to the basin or watershed. But cloud seeding will not work during a dry spell where there is a shortage of the right storm clouds.
According to the Tahoe Tribune, cloud seeding has been used in the Sierra Nevada mountains for decades. The article says, “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.”
California and Nevada are in the semi-arid western United States, where it is normal to have long periods of drought during the Spring, Summer and Fall, but healthy storms during the Winter months. 2012 had everyone concerned because of a record breaking mild winter. But some good storms have come up during March and April. Lake Tahoe, which sits on the eastern side of the Sierras, is the perfect “basin”.
The bad news about cloud seeding is that the procedure will not create clouds to bring an area out of a drought. Albert Huggins is the associate research scientist at the Desert Research Institute in Reno, Nevada. Scientific American asked him if cloud seeding helps with drought. Huggins said,
“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.”
The best conditions for cloud seeding include a 9,000 foot storm altitude, 23 degree Fahrenheit temperatures, moisture that is colder than freezing temperature and not too much wind. If airborne release is the seeding method, the cloud structure has to be high enough off of the ground for an airplane to get to the right point.
Two methods are used: one method is to blast the ice nucleating substance from the ground and into the sky ahead of the storm. The second procedure is to fly into the storm and release the bullets.
In summary, with the right storms and the right basin, it is possible to use cloud seeding to produce more ice crystals and thus, more water. Cloud seeding has not worked where there are no clouds or where there are the wrong type of clouds. Cloud seeding has been in used with success in parts of the world where the proper storms can be exploited.