December 26, 2014--Biggest cloud-seeding experiment yet only sparks more debate (Live Science)

Meteorologists first conceived of seeding clouds as a way to increase rainfall in 1946, working at General Electric's laboratories in Schenectady, New York. But in the nearly 60 years since then, it has remained unclear whether human attempts to make it snow actually work. Now, the results of the most scientific study of cloud seeding done yet are in. Researchers found that seeding clouds with droplets of silver iodide does slightly increase precipitation, boosting levels by 5 to 15 percent. However, experts disagree about whether this small increase means cloud-seeding efforts should expand. In Western states, water providers, ski areas and power companies interested in hydroelectric generation have all injected silver iodide droplets into winter clouds for decades. In those areas, the winter snows that collect on mountain ranges provide upward of 70 percent of annual precipitation. The idea is that the droplets provide a nucleus within a cloud around which water can coalesce, forming snowflakes. A 2003 National Research Council report called "Critical Issues in Weather Modification Research" sharply criticized the core idea. Although human activities can clearly affect weather, "there is still no convincing scientific proof of the efficacy of intentional weather-modifications efforts," the committee wrote in the report. The problem, the report added, was "the absence of adequate understanding of critical atmospheric processes that, in turn, leads to a failure in producing predictable, detectable and verifiable results." Researchers in Wyoming accepted the challenge of finding such verifiable results. They conducted a $14 million randomized blind statistical experiment that was designed and evaluated by the National Center for Atmospheric Research (NCAR). The primary laboratory consisted of two parallel mountain ranges, the Sierra Madre and Medicine Bow. During six winters, from 2008 into 2014, the researchers seeded storms that blew over both mountain ranges.

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