A nine-year, $14 million study says that cloud seeding can increase snowfall, and some officials in thirsty Arizona say it’s time to ramp it up.

The study is the most ambitious and expensive ever done on the practice, which involves adding silver iodide to clouds to increase precipitation. It concluded that — despite several caveats and uncertainties — seeding in two Wyoming mountain ranges increased snowfall by 5 percent to 15 percent and raised stream flows by 0.4 percent to 3.7 percent.

If that’s true, cloud seeding could improve Colorado River Basin water flows, some water experts say. The river is Tucson’s main source of drinking water, which the Central Arizona Project brings 336 miles uphill via a concrete canal and pipeline.

Based on the study’s results, a Central Arizona Project official said he hopes to start full-scale discussions of planning for a regional seeding program after the current snow season ends. But some water experts say the study had too many glitches to justify launching a regional program so soon.

Chuck Cullom, CAP’s Colorado River programs manager, called cloud seeding a “reasonable and viable” strategy for the Colorado River system. Daniel Breed, a scientist for the National Center for Atmospheric Research, which monitored the study, agreed that it’s time to start planning a regional seeding program.

Cullom envisions producing from 250,000 to up to 500,000 acre-feet of water annually at $10 million to $12 million a year. That’s about 3 percent to 4 percent of the Colorado’s total average annual flow since the region kicked into a drought mode in 2000.

The new study was discussed in detail at a national conference of the American Meteorological Society here. Debate was spirited.

Dave Reynolds, a longtime meteorologist and former cloud-seeding researcher, pronounced it a scientific failure because the results didn’t pass a standard statistical significance test. Reynolds is doing a literature review of climate seeding efforts under contract with the U.S. Bureau of Reclamation.

Michael Manton, an Australian researcher who served as a consultant on a seeding study there that produced similar results, said more study is needed to ensure cloud seeding is efficient and effective. But he agrees it’s been shown to work.

Terry Deshler, a Wyoming researcher who advised the state on the new study, said advocates’ optimism about cloud seeding is premature.

“Of course the water managers and developers ... already start to plan how to use the extra 15 percent of snowpack over the winter. They seem to overlook that maybe it’s only 5 percent,” said Deshler, a University of Wyoming atmospheric science professor.

CAP’s Cullom noted that even skeptics haven’t said cloud seeding doesn’t work.

“The people with a dissenting view simply are not as confident or would like to see a higher level of confidence than we have,” he said.

THE STUDY’s origins

The Wyoming study comes nearly 70 years after the possibilities of cloud seeding were discovered by accident and more than a decade since the National Academy of Sciences concluded that cloud seeding’s benefits were unproven.

The 2003 report from the academy’s National Research Council noted what it called a paradox in the weather modification field. There were at the time 66 individual cloud seeding programs ongoing in 10 states. (Nine Western states are doing cloud seeding on their own today.) But federal support for weather-modification research had dropped from $20 million annually in the late 1970s to less than $500,000 a year by 2003, the report said. Today, the feds provide little if any cloud-seeding-research support.

A central problem the academy cited is that no past cloud-seeding experiments verified that the technique actually produced additional rainfall or snowfall.

“A Catch-22 ensues in which the inability to provide scientific proof damages the credibility of the entire field,” leading to even less research to find solutions to this problem, the report said.

“We know that human activities can affect the weather, and we know that seeding will cause some changes to a cloud. However, we are still unable to translate these induced changes into verifiable changes in rainfall, hail fall or snow on the ground,” concluded the report, the fourth the academy has written on cloud seeding since the 1960s.

Wyoming officials said they designed their study to meet the academy’s concerns. They randomly seeded in two high mountain ranges — the Medicine Bows and Sierra Madres — alternately using one range for experimental seeding and the other as a control where no seeding was done.

They allocated $4.5 million of the $14 million tab to hire an outside reviewer, the National Center for Atmospheric Research. They used computer modeling on a large scale for the first time in cloud seeding research. It was to simulate seeding’s impacts and to predict cross-contamination, in which silver iodide sprayed into one range found its way into the other.

About $300,000 of the study’s cost came from CAP ratepayers and property-tax payers, and from revenue from sales of surplus power from the Hoover Dam. Wyoming paid most of the cost, with California and Nevada water agencies picking up the balance. “We had two separate contracts: one for operators and one for evaluators, so we don’t have the operators evaluating their own results,” said Barry Lawrence, the Wyoming Water Development Commission’s project manager on the study. “And as a check to those guys, the University of Wyoming checked on them.”


Each of 154 seeding operations, conducted over six years of the nine-year study period, lasted four hours. Computer-model tests, which sought to simulate three years’ worth of field research, predicted precipitation would increase 10 percent to 15 percent.

But when looking at on-the-ground results, researchers first concluded that seeding had increased precipitation an average of 3 percent — and found a 28 percent probability that the results occurred by chance. Typically, scientists want no more than a 5 percent likelihood of a chance result.

The study’s success rate improved to 15 percent when researchers discarded seeding efforts in which silver iodide sprayed in one range blew to the other, and efforts in which generators malfunctioned.

But University of Wyoming researchers found another glitch. Only 30 percent of the time were the clouds seeded at the right temperatures and was wind blowing in the right direction to achieve desired results.

Overall, the study concluded the raw results weren’t statistically significant, but that modeling studies, other research techniques and related studies led to the conclusion that cloud seeding is viable.

CAP’s Cullom noted that the seven to eight years of monitoring in this study is unprecedented, and that its use of two mountain ranges removed “observer bias.”

The research’s lack of statistical significance isn’t a deal breaker because, with natural systems like weather, it’s difficult to achieve results of which you’re 95 percent confident, Collum said. This study’s 78 percent rate, he said, “is a significant achievement over past efforts.”

Australian researcher Michael Manton said researchers still need to learn more about the character of clouds and storms to make sure they’re seeded most efficiently, and that trying to seed them all is an inefficient use of money.

“We have come a long way” toward meeting the academy’s concerns, but “we’re not there,” he said.

But Roy Rasmussen, a senior scientist for the National Center for Atmospheric Research, said he believes the academy will be proud of this study, since the researchers followed its advice in many areas.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. Follow Davis on Twitter @tonydavis987