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Weather modification study shows no extra-area impacts from cloud seeding

by Wyoming Livestock Roundup

Cheyenne – December 2014 marked the completion of the nine-year Wyoming Weather Modification Pilot Program (WWMPP), and the results of the study showed positive results from cloud seeding without negative impacts that some expected. 

“A common question that gets asked of the cloud seeding research committee is, if we increase precipitation in one place, what happens in other places?” said Sarah Tessendorf, project scientist with the National Center for Atmospheric Research (NCAR). 

Using a model-based study, Tessendorf looked at the impacts of cloud seeding and stated, “Our general conclusion is the extra-area effects were small and near zero.”


The models proved and utilized by other aspects of the WWMPP were put to work to determine if extra-area effects could be seen as a result of cloud seeding. 

“Modeling can be a useful tool,” Tessendorf noted. “We modeled the whole state of Wyoming to capture what effect might happen.”

Models particularly look at the Wind River, Sierra Madre and Medicine Bow Mountains at a very high resolution. 

“We ran a simulation for a whole month to determine what longer-term effects might be,” she explained. 

By programming in weather data from the years of the WWMPP, the NCAR team looked at precipitation and what it might have been. 

“We identified opportunities for seeding over that simulated month and how many storms were seed-able,” Tessendorf continued. 


After running the model, Tessendorf noted, “We tend to see larger positive effects of precipitation after seeding.” 

Increases in precipitation in the target area re-affirms the positive impacts of cloud seeding. 

The WWMPP Executive Summary stated, “Given the observational constraints of the WWMPP, there were no measurements to validate the model beyond the intended seeding target areas, and therefore, these results should be interpreted with the caveat that they were based on model results.”

However, WWMPP also noted that the effects cause less than 0.5 percent change in precipitation.

“In the extra areas, we get a scattering of small positive and negative effects,” she continued. “We saw a difference of a few millimeters over a few months. These are very small and near-zero effects outside of the target area.”

Particularly encouraging, Tessendorf noted that the results of the model were consistent with those found in studies conducted in 2001 and 2014.

Environmental impacts

Another concern related to the WWMPP was the environmental impact of seeding as a result of the seeding agent, silver iodide. 

“Silver iodide has a minimal impact,” said Tessendorf, noting that the impacts are markedly smaller than naturally occurring levels of silver in soils. 

The WWMPP Executive Summary said, “Trace chemistry analyses of water and soil samples were conducted for all three ranges following each operational season.”

The analyses noted negligible environmental impacts, with silver concentrations in water ranging in the parts per trillion and soil concentrations in the parts per billion. 

“These concentrations are far less than would be expected from other potential background sources of silver,” added the Executive Summary, “and measured concentrations in water sources were about three orders of magnitude less than values considered hazardous to environmental systems or human health.”

For more information on the increase in snowpack and streamflow as a result of cloud seeding, check out the Dec. 10, 2014 edition of the Wyoming Livestock Roundup. 

Saige Albert is managing editor of the Roundup and can be reached at

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