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Weather modification pilot project continues in three Wyoming mountain ranges

by Wyoming Livestock Roundup

The Wyoming Weather Modification Pilot Program (WWMPP) is wrapping up its sixth winter of a cloud seeding study, and hopes to continue the program through the next two winters.
Cloud seeding is the act of creating additional snowfall from clouds through injecting those that contain super cooled liquid water (SLW), and that are in a temperature range that permits efficient ice nuclei activation, with a silver iodide/acetone smoke created through burning propane.
“This silver iodide smoke gets into these cloud and creates (hopefully) the snow you see on the ground,” explains Roy Rasmussen of the National Center for Atmospheric Research (NCAR).
“After the initial five-year pilot program we approached the legislature and received $2.85 million in additional funding last year. That provided us with enough funding to finish this year, and next year’s winter season. We intend to approach the legislature again next year to ask for funding for one more year, so we have eight years in the project. It’s all about getting enough cases to reach statistical significance at this point,” explains Wyoming Water Development Commission Program Manager Barry Lawrence.
He adds that the first two years of the program, 2005-2006 and 2006-2007, were spent getting permits drawn up, completing exploratory studies, installing equipment and reviewing the scientific experimental design.
The last three years of the program have been spent collecting data in two target areas, in hopes of reaching numbers that will be of statistical significance. These areas include the Medicine Bow Range and the Sierra Madre Range in southern Wyoming as one target area, and the Wind River Mountains in west-central Wyoming as another.
Seeding is done using 26 ground-based generators developed by Weather Modification Incorporated (WMI).
“The generators burn a seeding solution, producing a fine aerosol that is then ingested by the clouds. The burner assembly consists of a shroud that houses the propane burner, atomizing nozzle and igniter. The silver iodide solution flows to the atomizing nozzle where it is sprayed into a propane flame. A series of valves controls the flow of propane, nitrogen and seeding solution. Solar panels provide DC power and remote capability is provided through a control computer and satellite communication link. The network of generators is controlled by a centrally located computer, where the user can control and monitor the performance and status of each generator,” says WMI.
“One of the ways we determine if the smoke is getting into clouds and creating additional snowpack is with radiometers, which look like a big mailbox. These are a way to remotely detect whether there is SLW in the cloud. We have three radiometers deployed: one upstream of the Medicine Bows, the Wind Rivers and the Sierra Madres,” explains Rasmussen. “In addition, we at NCAR are running a numerical model. This is a state-of-the-art weather forecast model we run every three hours, and we give the forecasters forecasts of where the winds are going, what the temperature is going to be and what kind of SLW we will have over the ranges. That information can then be used to help them decide on cases.”
Weather balloons are also used to determine when to start a case. Each case consists of a four-hour seeding period with a “buffer” period of four hours to clear the areas of seeding materials, which become one statistical unit. Forecasters gathering data on the resulting snowfall are not told which generators were turned on for any given case, to ensure their results are unbiased.
“We initially saw the potential to increase snowpack by 10 to 15 percent. We had 84 cases going into this season, and had 101 cases by February 2011. We need 166 cases for statistical significance. We average about 30 cases a year, so that’s why we feel we need two more years,” notes Lawrence.
He adds that, as of press time, cloud seeding had been suspended in the Sierra Madres and Medicine Bows because of snow water equivalent amounts in the target areas. Cases are still occurring in the Wind River Mountains, which have had a total of 15 events for this winter. This winter the Sierra Madres and Medicine Bows split 24 events roughly half-and-half between them prior to suspension.
“The degree of confidence with the numbers is not where it needs to be to start releasing that information,” explains Lawrence of the reason why data showing snowpack results for the project’s completed years isn’t yet public, and why those involved in the project are continuing to work toward enough cases to reach statistical significance.
“I can tell you that trends are developing, and, as of today, they are positive and encouraging for cloud seeding as a means for snowpack augmentation,” he adds.
Lawrence and Rasmussen spoke during the Wyoming Farm Bureau Federation Legislative Meeting in Cheyenne on Feb. 15. Heather Hamilton is editor of the Wyoming Livestock Roundup and can be reached at

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