New cloud seeding study from Wyoming

Research renews the debate about whether scientists can really make more rain.


If only clouds could be more efficient in bringing us rain, our water supply woes might be lessened. Scientists have been working on how to modify the natural process of precipitation since 1946, when chemist Bernard Vonnegut (brother of Kurt Vonnegut) declared that injecting clouds with silver iodide could change how they behave.

Since then, most Western states have tried cloud seeding at some point, in the hopes of giving watersheds, farms and even ski resorts a boost. And yet, there’s never really been a consensus in the scientific community about how well cloud seeding truly works.

But last week, the 16-page summary of a long-awaited study that aims to quantify the impact of cloud seeding was released, renewing debate about whether the practice can in fact bring more rain and snow. The research is a product of the $13-million-dollar Wyoming Weather Modification Pilot Project to study snowfall over six winters in southern Wyoming, and may be the most extensive study of cloud seeding yet.

The report suggests that, during ideal conditions, a seeded cloud will produce between 5 to 15 percent more precipitation. The practice is a viable way to augment existing water supplies, researchers say. They also found that cloud seeding has a negligible environmental impact and can be cost effective. Marc Pitchford, the executive director of the atmospheric science division at the Desert Research Institute, a weather modification company, says that the new data may help open the door to further research: “Existing cloud seeding programs may be taken more seriously (now) by water managers and agencies affected by ongoing drought conditions.”

Seeding is most commonly done by heating and releasing microscopic silver iodide particles into approaching storms, either from ground-based generators or by airplane. The silver iodide enters the cloud, and the theory goes that the chemical acts as ice-forming particles where there otherwise weren't any. This boosts the natural process as ice crystals bond to the chemical, weighting the cloud and releasing precipitation.

Despite the report’s optimistic findings, scientists are stressing that cloud seeding won’t be a quick fix for water scarcity. Critics are eager to point out that the 5 to 15 percent jump in rain and snow is for clouds that already have ideal conditions, like proper temperature, moisture or wind speed. Looking at all storms over the test period, the precipitation increase is more like 3 percent, the study says. And the ideal conditions were absent from about 70 percent of the storms studied over six winters. “You can’t apply that 5 to 15 percent to (all of the) snow over the winter because more than half of the snow comes from storms that would not necessarily be identified as seedable,” Deshler said.

The report also looked at how much water could be added to a watershed, if cloud seeding bumped normal precipitation by 10 percent. Over the North Platte River Basin in Wyoming – an area that covers about a quarter of the state – that would mean an additional 7,100 acre-feet of water, or a 1.8 percent annual increase.

The new research, which was funded by the Wyoming Water Development Commission, and was a collaboration of the University of Wyoming, several weather modification companies and weather research institutions, shows that the method can indeed give us more rain. Yet plenty of questions remain: Whether seeding works will still depend on weather and climate of any given region. “The difference between a wet winter and a dry winter is not going to be found in the precipitation caused by cloud seeding,” said Bart Geerts, a professor of atmospheric sciences at the University of Wyoming.  And yet, “a lot of scientists believe that even if there is a chance for a 1 percent increase (in annual precipitation), it is a worthy endeavor, given current water scarcity.”

The full report is expected to be published in May 2015.

Dave Zook is a guest contributor to High Country News. 

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