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Wyoming, UW Finish Collecting Cloud Seeding Statistical Information

April 4, 2014
Man working with weather equipment
A Weather Modification Inc. technician mans a silver iodide cloud-seeding generator in the Medicine Bow Range. A cloud-seeding research project originally commissioned by the state of Wyoming nearly a decade ago and involves the University of Wyoming has been completed -- at least on the statistical gathering front.

A cloud-seeding research project originally commissioned by the state of Wyoming nearly a decade ago and involving the University of Wyoming has been completed -- at least on the statistical gathering front. Now, it’s time to crunch the collected data before results are presented to state legislators at the end of this year.

The compiled data should help to answer the $100,000 question: Does cloud seeding work or not?

“Wyoming contracted with a cloud seeder (Weather Modification Inc.) and a seeding evaluation contractor,” says Terry Deshler, a UW professor in the Department of Atmospheric Science and a member of the project’s Technical Advisory Committee. “The goal was to run a scientifically credible experiment with an evaluator to statistically test the effect of cloud seeding. That’s been an experiment going on now for eight years. Now, we go into data quality control and the analysis phase.”

With the data collection phase completed, actual results -- now being compiled for verification and testing by scientists and statisticians with the National Center for Atmospheric Research (NCAR) -- will not be made publicly available until December, when a final report is given to Wyoming’s legislators and a scientific paper is submitted for publication.

“We have a rough outline. We don’t know the results,” Deshler says. “NCAR will be leading the paper, and all funded partners will be writing pieces of the report. Other experts and I will review the paper and the report. Barry Lawrence (project director for the Wyoming Water Development Office) will see everything. We will not be talking to people about this prematurely.”

The Wyoming Weather Modification Pilot Project (WWMPP), which began in 2006-07, technically was set to conclude at the end of April 2014. However, the cloud-seeding project was halted April 2 due to above-average snowpack (120 percent of the 30-year average) in the mountains of southern Wyoming. The Wyoming Water Development Office made the decision as a precautionary measure against exacerbating the potential of spring flooding.

"We don't want to exacerbate anything that could be coming down the road," Lawrence says in recent media reports. "It's not worth continuing on, so we're pulling the plug."

Before the decision to halt the project, Deshler says it was possible a few more cloud-seeding experiments could have been conducted this month, if snow conditions were ideal.

Planting the seeds

In 2005, the Wyoming State Legislature approved a five-year weather modification study administered by the Wyoming Water Development Office. The state-funded program is designed to evaluate the effectiveness of cloud seeding, a form of artificial weather modification, with silver iodide to enhance snowfall from winter orographic storms in the Medicine Bow and Sierra Madre mountains. Such storms occur when an air mass is forced from a low elevation to a higher one as it moves over rising terrain.

Due to water shortages and droughts in some states and in countries around the world, cloud seeding is seen as a potential way to increase water supplies for communities and to irrigate crops. Cloud seeding typically is paid for by water resource managers, power companies (hydropower) and agricultural interests.

“This experiment by the state of Wyoming is the first statistically valid experiment that has been run in the U.S. on cloud seeding since the 1960s to the ‘70s,” Deshler says. “People gave up (after that time period) because of the amount of time required to do cloud-seeding experiments.”

Deshler is a co-writer of a paper, titled “Evaluating Winter Orographic Cloud Seeding: Design of the Wyoming Weather Modification Pilot Project (WWMPP),” that was published in the February issue of the Journal of Applied Meteorology and Climatology.

The paper compiles the parameters set forth in advance for the Wyoming cloud-seeding study.

“As a scientist, you want to say at the beginning how you plan to run your project,” Deshler says. “It lends the project credibility.”

Precipitation in winter orographic storms generally develops when ice crystals form and grow on natural ice nuclei, such as dust particles. In many storms, the lack of natural ice nuclei active at warmer temperatures results in an inefficient precipitation process. In addition, weak updrafts in these clouds and narrow cloud droplet distributions limit the impacts of any ice processes to multiply.

During cloud seeding, silver iodide is released into the clouds through generators that 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.

Testing period

Randomized cloud seeding began during 2008-09, with the seeding period running from Nov. 15 through April 15 each year. Since the project started, the Legislature has twice funded two-year extensions to the original five-year appropriation because the number of cloud-seeding experiments was not sufficient to reach statistically significant conclusions for the WWMPP, Deshler says.

The original expectation was to run 65-70 four-hour experiments during the seeding period for each year. However, based on conditions ripe for cloud seeding, experiments each season numbered in the 25-30 range, Deshler says. Over the course of six years, that would mean 150-180 experiments were conducted.

“Is that enough (experiments) if we assume some change?” Deshler asked rhetorically. “That depends on the magnitude of the effect. Small changes will limit the confidence we have in the result because of the large variability in natural snowfall. An effect of 10 percent may be difficult to substantiate with confidence, based on the number of experimental cases. But, let’s wait until the data are analyzed later this year.”

“After we analyze all the data, we’ll get a distribution of effects that gives us a mean,” he adds. “That mean is the overall result of this experiment. Along with the mean, we will have a statistical confidence in the validity of the mean. We had hoped to achieve 95 percent confidence initially. Now, we’re not sure it will be that high. But that depends on the data we have. So, we’ll see.”

Initially, the Medicine Bow and Sierra Madre ranges were going to be treated independently relative to cloud seeding decisions, with results to be pooled for statistical evaluation. However, numerous factors suggested storm conditions and precipitation would be far from independent in the two ranges, Deshler says.

There was a correlation of about 50 percent between the ranges for all storms, which affected snowfall. This correlation made pooling more difficult, as many experiments in the two mountain ranges would not be independent.

As a result, the researchers decided to design an experiment using a randomized cross-over design. Using the two cloud-seeding target areas, paired data can be produced that is more efficient and decreases sample size. Sticking with a single-target design for each area could require 15 or more years of data, Deshler says.

“We don’t have that kind of time or funding,” he says.

During each experiment, a buffer period up to four hours following cloud seeding was used, according to the research paper. The buffer period was used to guard against contamination, meaning researchers wanted time to determine whether seeding material may inadvertently affect precipitation in an area considered to be unseeded.

The cloud-seeding estimates are highly dependent on the expected effect of seeding, on correlations between the two mountain ranges and between the target and control sites, and on the variance of precipitation.

The paper’s other co-writers are Roy Rasmussen, senior scientist and deputy director of NCAR’s Earth and Sun Systems Laboratory; Daniel Breed and Courtney Weeks, both with the title of associate scientist II in NCAR’s Research Applications Laboratory; and Bruce Boe, director of meteorology of Weather Modification Inc., a Fargo, N.D.-based cloud-seeding company.

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