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Study: Cloud Seeding Really Does Work to Artificially Induce Snowfall

Cloud seeding has been going on for decades as a tool to boost water supplies. Only now has a scientific study proven the process actually works. But as atmospheric scientist Jeffrey French explains, another big question looms: How effective is it?

Written by Matt Weiser Published on Read time Approx. 5 minutes
Gettyimages 24074781
Aircraft technician Steve Bauer of Weather Modification inspects wing-mounted silver iodide flare racks on a Piper Cheyenne II aircraft before the beginning of cloud-seeding operations. The company, based in North Dakota, has contracts to conduct cloud-seeding operations for water agencies, utilities and state governments.NDRANIL MUKHERJEE/AFP/Getty Images

Cloud seeding has become big business worldwide as a means to boost water supplies. Utilities and governments spend tens of millions of dollars on the process, which is especially common in Western states that rely on winter snowpack to meet year-round water demand.

The basic process involves spraying silver iodide from a plane as it flies through storm clouds. The silver iodide induces moisture in the cloud to form ice crystals, which then (hopefully) fall out as snow.

Some studies have estimated cloud seeding can boost snowfall by between 8 and 15 percent. This figure was derived by comparing snow depth on mountains beneath clouds that were seeded, compared to nearby mountains in unseeded areas affected by the same storm. And it was deduced that seeding made the difference.

But amazingly, the basic physical process believed to occur during cloud seeding has never been conclusively proven. No scientist has ever verified that silver iodide causes ice to form in a cloud, and that the artificially created ice then reaches the ground as snowfall.

Until now. In a new study, a team of scientists led by Jeffrey French at the University of Wyoming in Laramie has proven the entire chain of events, from ice formation in the cloud to snow accumulating on the ground as a direct result.

The study does more than simply prove conventional wisdom. French, an assistant professor of atmospheric science, tells Water Deeply that dissecting and verifying the process will help make cloud seeding more effective.

Water Deeply: What did you prove in this study, exactly?

Jeffrey French: What we showed was that in certain conditions, when you add silver iodide to a cloud, you can get the cloud to nucleate ice particles that otherwise would not. So you’re freezing some of the supercooled liquid that otherwise would remain a supercooled liquid, in conditions that would then allow those newly formed ice crystals to grow through a variety of natural cloud processes to a point that they are large enough to then fall out of the cloud and land on the surface of a mountain as snow.

We were able to document that entire process, and that’s never actually been done before. Nobody has ever been able to probe into it repeatedly with time and look at the evolution of the cloud particles. And that’s what we were able to do.

Water Deeply: Why is it important to verify this process?

French: It’s important to be able to evaluate whether cloud seeding is really having an impact. The big question is, at the end of day, are you putting more snow on the ground in any significant amount? It’s a statistical question and it’s an area question. It’s done through looking at correlations, between times when you seed and how much snow falls and times when you don’t and how much snow falls. The problem with that approach is that there is so much natural variability in the world that you can have what appears to be a positive signal, but it may come up just by pure chance or there may be other impacts that cause you to have more snow at these times versus other times.

Jeffrey French, assistant professor of atmospheric science at the University of Wyoming, with the King Air research plane used in his study of cloud seeding. (Photo Courtesy University of Wyoming)

The second question we addressed was this physical connection. There’s a physical hypothesis in terms of what happens when you put silver iodide in a supercooled cloud. If you can’t validate that physical hypothesis, then you don’t really have anything to stand on in terms of evaluating the overall effectiveness of cloud seeding.

There’s still a lot of research that needs to be done.

Water Deeply: Will this lead to improved practices in cloud seeding?

French: Certainly. I would say yes, emphatically, about that. The very first part of this research – understanding what the physical mechanisms are and the conditions under which they may have a larger impact on a cloud – is really important in terms of deciding what clouds you want to target, for example.

Water Deeply: How did you prove it works?

French: We looked at one part [of a] mountain range, the Payette Mountains north of Boise, Idaho. We worked with Idaho Power Company, which has an operational cloud-seeding program.

They would send their aircraft upwind of that mountain range, flying back and forth at a constant altitude and releasing silver iodide. Then we had a number of instruments on the ground. We had a couple of research radars parked on mountaintops for the entire winter season. When conditions looked conducive for cloud seeding, a crew jumped on the radars and started operating them and they would stay up there for a week or so while these storms came through.

Then at the same time we had a research aircraft owned and operated by the University of Wyoming and funded by the National Science Foundation. It’s modified and instrumented with its own radars and LIDAR [a surveying method using pulsed laser light], and it’s got a number of very sophisticated probes and instruments that hang off the nose and wings that can measure in very fine detail individual cloud particles as we’re flying through the clouds. The research aircraft is flying in and out of the seeded area. That allows us, then, to compare the character of the cloud particles we measure in regions that are seeded versus regions that are not.

Water Deeply: How important is cloud seeding these days?

French: If you look at the mountainous western U.S. that includes the states of Montana, Wyoming, Colorado, New Mexico and everything to the west of that, all of those states with the exception of Oregon and Washington have active cloud-seeding operations occurring in them. So there’s a lot of money being spent on cloud seeding. The question is up in the air, in my mind still, how much of an impact it is having on water in the West. I don’t think anyone can really answer that question scientifically at this point.

Water Deeply: Do you plan to answer that question?

French: We do want to answer that question.

The fact that we don’t know the answer is not because of lack of effort on anyone’s part. An awful lot of research was done in the 1960s, ’70s and ’80s on this. A lot of huge, great discoveries just in terms of cloud physics were made during this time. But they were never able to answer this question and that was kind of when the research went away. It also coincided with a time in the West when water was not a huge issue. It was a relatively wet period.

We’re able to make so much better measurements now, in terms of understanding the microscopic details of what is occurring in a cloud, that now is a good time to revisit some of that research and try to go back out and make the same measurements again that they were trying to make 20–30 years ago.

If nothing else, what we learned from this study is that by taking this improved technology in terms of instrumentation and measurements, we can still make significant breakthroughs. Our experimental design, outside of the instruments and measurements we had available to us, was not significantly different from some of the experiments that were run in the 1970s and 1980s. We just have much better tools now to make measurements.

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