The University of Wyoming played a key role in a nearly decade-long study by the state that concluded cloud seeding has some effect, with a five to fifteen percent positive impact on snow accumulation shown after the weather modification technique is used.

However, UW researchers were cautious in their comments on the findings, which were presented to the Wyoming Water Development Commission at that board’s meeting in Cheyenne on December 10.

While the overall draft executive summary report concluded that cloud seeding is a strategy for long-term water management in Wyoming, Terry Deshler, a retired UW professor of atmospheric science, stresses that not all winter snowstorms are conducive to using cloud seeding. During cloud seeding, silver iodide is released into the clouds through generators. In the case of the state’s $13 million study, generators were strategically placed upwind of the ridges of the Medicine Bow and Sierra Madre mountains in southern Wyoming. The silver iodide facilitates ice crystal formation in super-cooled water clouds.

“Not all winter precipitation is seedable,” Deshler told board members during his portion of the presentation on the draft study results of the state’s nine-year Weather Modification Pilot Program. “Estimates of seeding impact cannot be applied to the entire winter snowpack.”

Deshler said that seeding of wintertime clouds is appropriate only under certain meteorological conditions.

“Considering only conditions when precipitation was occurring during seedable conditions indicates, on average, 30 percent of the wintertime snowpack over the Medicine Bow and Sierra Madre ranges for the years 2000-2008 would have been seeded under the conditions specified for the RSE (randomized statistical experiment),” says Deshler, a member of the cloud-seeding project’s Technical Advisory Committee.

“NCAR (National Center for Atmospheric Research) is suggesting snowpack increases of five to fifiteen percent for seedable storms. But, what percentage of winter storms are actually seedable?” says Barry Lawrence, project manager of the Wyoming Water Development Office. “Terry Deshler has kept us credible.”

Deshler’s work shows that the five to fifteen percent increase due to seeding translates to a one and a half to five percent increase in snowpack.

A final report is expected to be presented to the Wyoming Water Development Commission in March, says Lawrence. A draft executive summary of that report was presented this month because researchers wanted to present their findings to the American Meteorological Society in January, when that group hosts its annual meeting in Phoenix, Arizona.