Cloud seeding has been done since before the 1950s , with the intent of increasing precipitation. But does it work? Research seems to indicate that yes it probably does, but only a little, and then only on average.
http://fivethirtyeight.com/features/we-may-never-know-how-well-cloud-seeding-works/
http://fivethirtyeight.com/features/we-may-never-know-how-well-cloud-seeding-works/
After nearly a decade of work, scientists concluded that cloud seeding could boost precipitation by 5 to 15 percent. The evidence came from an experiment in which two adjacent mountain ranges in southern Wyoming — the Medicine Bow and Sierra Madre — were randomly selected for seeding when conditions were amenable. Given the close proximity of the two ranges, they’re often hit by the same storms, so the unseeded range could serve as a control when the other was seeded. Researchers measured the results with snow gauges located in the target areas, as well as in areas outside the seeded zone for comparison. In addition, they analyzed snow samples to find out if the silver iodide was present in snow and whether it was accumulating in the environment in significant levels. (The answer to the latter question was no.) The researchers also used high-resolutionWeather Research and Forecasting modeling to simulate seeding operations.
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It’s important to put the numbers in context, says Deshler, the University of Wyoming scientist. When he and a graduate student looked at climate data on these mountain ranges over the past eight years, they found that only about 30 percent of the precipitation that fell came from storms that had the right temperatures and wind directions to be seeded. If you assume that you can get a five to 15 percent bump in precipitation out of those storms, you’re now looking at a total increase of precipitation of only about 1.5 to 4.5 percent, he says. “Water managers need to be realistic about what the real benefit is. You can’t take that 15 percent and say that’s 15 percent more snow over the winter.”