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Weather modification – WWDC releases Weather Modification Pilot Program results

by Wyoming Livestock Roundup

Cheyenne – A nine-year Level II Study by the Wyoming Water Development Commission came to a conclusion this year, and the Wyoming Weather Modification Pilot Program (WWMPP) released its draft executive summary at the Dec. 10 Wyoming Water Development Commission and Select Water Committee Workshop. 

The results, compiled from a wide variety of sources and with the help of numerous contributors, were largely positive, indicating that weather modification and cloud seeding was successful in the Sierra Madre and Medicine Bow Ranges.

Roy Rasmussen, senior scientist at the National Center for Atmospheric Research (NCAR), said, “We looked at physical, randomized and modeling studies to verify cloud seeding works.”


Using data from a series of snow gauges strategically placed in both ranges, Rasmussen noted that NCAR was able to determine the impact of cloud seeding on snowfall. 

Over the course of the study, 154 cloud seeding events occurred. Rasmussen noted 36 of those cases were eliminated for data analysis as a result of quality control concerns regarding the snow gauge data or inoperable seeding generators. 

“We reduced the sample size from 154 to 118 cases after quality control,” he said. 

The 118 cases were analyzed using a regression ratio, and Rasmussen noted a result of 1.03.

“This indicated a three percent increase in precipitation from cloud seeding, with a 28 percent chance that the increase occurred by chance,” Rasmussen said. “That is a fairly large probability of it occurring by chance. We are looking for only a five percent chance increases are occurring randomly.”

Regardless, data suggests that cloud seeding works. 

Continued analysis 

“We did some further analysis and came up with two factors that occurred in our results,” he continued, noting unintentional seeding of the downwind range and insufficient generator hours, or not enough seeding material, may have had an impact on the seeding results. 

“When we seed one range, the other range serves as our control,” Rasmussen noted. “When we seed the Sierra Madres, which are to the west, the control may to be impacted.”

Because the Medicine Bow Range is downwind of the seeded Sierra Madres, some seeding effect may occur. 

The assertion was confirmed with higher concentrations of ice nuclei in the Medicine Bow Range when the Sierra Madres were seeded. 

“When we looked at measurements, we eliminated 21 cases because silver iodide reached the Medicine Bows in those cases,” he continued. “That changed our results from three to four percent increases.”

They further utilized modeling to track where the plumes of silver iodide were distributed. An additional 18 cases that indicated Sierra Madre seeding affected the Medicine Bows were eliminated, increasing the statistical impact of cloud seeding from four percent to nine percent increases in snow. 

Additional evidence

The number of generator hours was also considered. 

“It is important that we have sufficient numbers of generators covering the domain,” Rasmussen commented, noting that each of the eight generators running for the four hour seeding period results in a maximum of 32 generator-hours per seeding event. “We have a lot of days with less than 32 generator-hours.” 

He explained, “That suggests maybe we weren’t covering the area.”

The results  showed that including cases with less than 27 generator hours had a profound effect on the statistical analysis, reducing snow increases. 

“We also looked at the model itself and simulated each of the 57 events for the three years that were modeled,” Rasmussen commented, which was included in the final conclusions.

“Our final conclusion is that we see a five to 15 percent seeding effect in the Medicine Bow and Sierra Madres Ranges for seedable cases based on the criteria for which sufficient ground-based silver iodide seeding was achieved,” Rasmussen commented. 

Water impacts

With increases in snow, the increase in the available water, as well as the cost of that water, are also important. 

University of Alabama’s Glenn Tootle analyzed streamflow impacts  and cost. 

Using a model, which was proven in comparisons to actual data from the North Brush Creek watershed, Tootle analyzed water increases and costs based on five, 10 and 15 percent increases in snow during seedable events. 

Finally, the team looked at the impacts seen if between 30 and 80 percent of the North Platte Watershed was covered by seeding operations.

“We ranged it from 30 to 80 percent effective, as far as the area we can seed,” Tootle explained. “We worked with the Wyoming Water Development Office to find out how much it would cost to seed during that time.”


Tootle’s information showed that for a 10 percent increase in snow across 60 percent of the North Platte River Basin, an increase in 7,100 acre-feet of water per year would be seen, on average. That increase equates to a 1.8 percent increase in streamflow for the area. 

“If we look at the other extreme possibility, a 15 percent increase in snow covering 80 percent of the Basin, we see 14,000 more acre-feet of water,” says Tootle. “That is a 3.7 percent increase in water.”


Tootle’s team also developed a chart looking at the costs of increasing water, showing a range of costs depending on Basin coverage, percentage increase in snow and four options for the type of seeding program run. 

“When we looked at what we can do to look at the cost of a fully operational program, we have from the basic program, where we rent equipment and don’t do any analysis, to the Cadillac of programs, where we continue to monitor, evaluate and run models,” he said. 

“With a five percent increase of snow covering 30 percent of the basin, being the most extreme conservative example on the low side, we are talking $200 to $400 per acre-foot of water,” Tootle summarized. “Moving to the other end, if we see 80 percent of the North Platte River Basin and hit 15 percent efficiency during seedable events, we can get water costs down as low as $25 to $50 an acre-foot.”

He noted that values in between are most likely. 

Moving forward

After results were presented, the WWMPP executive summary recommended considering implementation of a cloud-seeding operational program after identifying five key concerns – barrier identification, program design, operational criteria, program evaluation and program management.

Each of the concerns is further explained in the report.

Wyoming Water Development Commissioner Margo Sabec of Casper commented, “I think this is a viable water development project that involves research, and that is where our money should be going.”

The possibility of implementing an operational weather modification program will be discussed at an upcoming meeting of the Wyoming Water Development Commission. 

Look for more information on the Wyoming Weather Modification Pilot Project (WWMPP) summary and results in an upcoming edition of the Roundup.

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

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