Cheyenne, Wyoming

Cloud seeding increased precipitation over the Medicine Bow mountain range in Wyoming.  Credit: NCAR

For the past six winters, meteorologists have sprayed silver iodide particles into storm clouds rolling over Wyoming’s mountains to see whether the technique can increase snowfall.

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The experiment was one of the longest-running and most rigorous tests yet of ‘cloud seeding’. An independent team of scientists now says that it worked — sort of.

Seeding the clouds squeezed 5–15% more precipitation out of them, says Roy Rasmussen, a meteorologist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, who led the evaluation team. But that statistic holds only if scientists eliminate parts of the test that went wrong, such as when the silver iodide spray did not completely cover the mountain range that the researchers were trying to seed.

Still, the team felt confident enough to recommend that the state of Wyoming set up an operational cloud-seeding programme, to build up the winter snowpack that serves as a crucial water resource for cities and farms. The state spent more than US$14 million on the initial test, which is the most substantial cloud-seeding research project in the United States in decades.

“The accumulation of evidence suggests that cloud seeding works,” Rasmussen says. “This will have a major impact on our field.”

Rasmussen helped evaluate the cloud seeding done by the company Weather Modification Inc. of Fargo, North Dakota. Representatives from various parts of the programme presented initial findings to state officials in Cheyenne, Wyoming, on 10 December.

Make it rain (and snow)

The concept behind cloud seeding is straightforward: put extra particles into a cloud to act as nuclei for water to condense around. Those extra particles then fall out as snow at the crest of the mountains.

But cloud seeding has a long and chequered history. Many projects simply spray silver iodide into clouds and claim it creates extra rain or snow. Of the projects that have been scientifically evaluated, most simply follow a single cloud or storm system and thus describe only a one-off event.

The Wyoming project is unique in running for six winters — building up enough cases to be rigorously analysed — and for having an independent team evaluate the results.

Notably, the test included a randomized control1. The team selected two mountain ranges that are near each other in southern Wyoming and that experience many of the same weather conditions. For each storm, one range was seeded while the other was left untreated. The scientists evaluating the project did not know which was which.

In each range, technicians set up a radiometer to scan for supercooled water in clouds approaching the mountains, which can indicate it is ripe for seeding. Then, if conditions looked right, they turned on eight remote-controlled generators that sprayed silver iodide along the upwind side of one of the mountain ranges for four hours. The eight generators at the second mountain range remained off. Technicians then measured the amount of snow inside and outside the intended seeding area.

Future plans

Over the course of six years, the team ended up with 118 cases of cloud seeding in one range or the other. Analysis of those alone was not enough to suggest that cloud seeding increased precipitation, Rasmussen says.

But then the team decided to drop some cases out of their data set. They eliminated the couple of times when silver iodide from the seeded mountain range had drifted as far as the unseeded control range, contaminating it. And they eliminated many cases where not all generators were turned on in the seeded range — for instance, because the winds were blowing in a direction that could have dumped extra snow on an important interstate highway.

“It’s really important to recognize the bias, but we think this is scientifically defensible,” says Terry Deshler, an atmospheric physicist at the University of Wyoming in Laramie who serves as an adviser to the project.

The NCAR team plans to present the findings in January at a meeting of the American Meteorological Society in Phoenix, Arizona. “This is one of the more detailed studies [of cloud seeding] that has ever been conducted,” Rasmussen says.

What happens next is up to the Wyoming legislature. At the Cheyenne meeting, lawmakers questioned the scientists on the costs of generating additional acre-feet of water, where they might store it and what the implications might be for downstream users in other states. Wyoming is part of the Colorado River basin, whose waters are bitterly fought over to serve the needs of tens of millions of people in seven US states and in Mexico.

Starting up the programme again would take as much as $725,000 per season, and a fair amount of work. The silver iodide generators in those two mountain ranges have been dismantled, although a third, unrandomized experiment continues in the Wind River range to the north.