Winter bonfires paying homage to snow gods have long been a tradition in cold weather regions around the world.

But in the last 70 years or so, communities in the western United States have gone beyond rituals and added a technological twist. Across hundreds of mountaintops, from the Sierra Nevada to the Sawtooths, Wasatch and Colorado Front Range, cloud seeding experts are now often burning small amounts of silver iodide with the aim of bolstering dwindling water supplies.

The vaporized metal particles are ideal kernels for new ice crystals. When moist, super-cooled air rises over mountain ranges under predictable winds, it sets up perfect conditions for the crystalline alchemy that creates snow, the white gold craved by ski resorts, ranchers and farmers and even distant cities that need mountain water to survive.

The scramble for water has intensified as global warming has battered much of the West during the last 20 years with heat waves, droughts and wildfires. With projections for declining snowpack and river flows, cloud seeding is becoming a regional climate adaptation measure costing several million dollars each year. In other regions, including parts of the central United States, seeding has also been used to try and enhance summer rains and to reduce the risk of severe hail storms.

Still Controversial

But even as governments and businesses increase the practice, questions remain about how effective it is, and some leading climate scientists say it should not be seen as a meaningful response to climate change. Though cloud seeding has expanded to cover tens of thousands of square miles across the West, there are no recent comprehensive studies assessing the effects in both targeted and non-targeted areas. Cloud-seeding permits are issued under a patchwork of state and federal rules, which sometimes cuts the public out of the process.

Several panels at a recent conference on weather modification addressed some of those issues, and included presentations on how efforts to cooperate on regional cloud seeding projects could serve as a model for governance of even larger scale climate-mitigating geoengineering projects.

Some elected officials and water experts say the money is well spent because it produces millions of gallons of water that can be stored and used during drier and hotter summers, when stream flows dwindle. Scientific studies in the last few years show that cloud seeding works during suitable weather patterns that are already conducive to snowfall.

In the winter of 2018-2019, water managers in Colorado said their central mountains cloud seeding program produced between 80,000 and 90,000 acre feet of water at a cost of $2.70 per acre foot—a bargain in a region where prices per acre foot can reach $30,000. An acre foot of water is 325,851 gallons, enough for two average Colorado families for a year.

Crystalline Alchemy

Most recently, a study by scientists at the University of Colorado, Boulder, quantified the snow production from cloud seeding in a 900-square-mile test area in Idaho, said Katja Friedrich, an atmospheric scientist and lead author of the paper. It appeared Jan. 22 in Proceedings of the National Academy of Sciences.

“Since we had no natural precipitation, we can unambiguously say that the water generated is solely due to cloud seeding,” she said. “We don’t necessarily produce a lot of snow, and it is expensive, but if regions are under severe drought everything should be on the table. While I don’t think cloud seeding will solve the problem, it should be one, amongst other, drought mitigation techniques.”

Previous studies have used snow measurements at specific points on the ground combined with computer models to estimate how much snow is produced by cloud seeing. In the new research, the scientists used a radar dish near the test area in Idaho to look inside the cloud. 

As the water vapor morphed into thickening ice crystals, the scientists’ computer screens showed a clear cloud seeding fingerprint tracing the paths of planes dispersing the silver iodide into the clouds. The tiny flakes were barely enough to coat an eyelash, the scientists said, but dusted the test area with about one-tenth of a millimeter of snow.

That might not sound like a lot, but multiplied across thousands of square miles, all winter long, it could add up to enough water to sustain one of Arizona’s desert cities through a hot summer.

Recent scientific papers about cloud seeding also acknowledge the growing interest in using cloud seeding to buffer warming and drying in some vulnerable parts of the world. A 2018 review of cloud seeding in the Journal of Applied Meteorology and Climatology, for example, said water will become scarcer “as a changing climate threatens the water volume of snow reservoirs in the western mountains.”

China and Israel used cloud seeding extensively even before some of the global warming-driven aridification trends became clear in the 1980s. Cloud seeding was also weaponized during the Vietnam War, as the U.S. sought to boost rainfall to disrupt supply lines through the jungle.

Some Scientists Have Misgivings

Research in the last two decades proves that the physical process works, but it’s still not clear how much water it produces. At best, the latest studies “hold promise for narrowing the uncertainty” that has accompanied such research over its long history, the paper concluded.

Altering clouds with plumes of metallic smoke might work on a localized basis, but it isn’t the right solution for large-scale and long-term regional drying and warming, said atmospheric scientist Kevin Trenberth, with the National Center for Atmospheric Research and the University of Auckland, in New Zealand.

“If I were concerned about water supply I would not invest in cloud seeding, but rather in management of water. Cutting down on evaporation from any reservoir, lake, river can be done. Most places have major management issues,” he said. “The water is not managed as a system or resource. I am not aware of any studies that show it to be beneficial overall.” 

At some point, if cloud seeding continues to expand, it could start to affect climate regionally, with impacts to areas downwind of the mountains and valleys that are the targets.

“Denver Water may capture more rain in the Rockies for their water supply at the expense of somewhere farther east,” Trenberth said.

Penn State climate scientist Michael Mann said there is a danger that cloud seeding could be seen as a meaningful response to global warming.

“When it comes to the larger impacts of climate change and the drying of the West, this amounts to rearranging deck chairs on the Titanic,” he said. “And yes, since it’s potentially a zero-sum game, you could indeed cause drying in some key regions, like the lee side of Rockies.”

He added, “Whether this is viable at regional scales and longer timescales remains to be seen. It is easy, for example, to imagine that seeding-produced precipitation in one location may have ultimately fallen later, downstream. This has the potential to lead to controversy and litigation over water rights. We can expect that conversation to heat up now, no doubt,” he said.

Global Interest

The natural variability of clouds makes it difficult to quantify the effects of cloud seeding, said Andrea Flossmann, a cloud researcher with the CNRS/Laboratoire de Météorologie Physique in Clermont-Ferrand, France. She led a 2019 review of global cloud seeding efforts for the World Meteorological Organization that recognized growing interest in the cloud seeding worldwide.

The WMO scientists concluded that global warming is driving conflicts over water shortages around the world, and that the increased pressure has spurred more interest in cloud seeding. A 2018 report from the international organization warned that some of the efforts are not science based and that some communities desperate for water might pay for programs that won’t deliver promised results. Since the process yields just a 10 percent increase under very specific weather conditions, it takes a lot of seeding to make it pay, Flossmann said.

It’s always hard for researchers to know how much snow would have fallen without cloud seeding, and that’s one of the reasons Colorado climate scientist Brad Udall, with the Colorado River Research Group, is not so sure about the current expansion of cloud seeding programs. Plus, the technique only works if there are clouds to begin with. It doesn’t help much during a drought, when natural precipitation is hard to come by, Udall said.

“I’ve been pretty skeptical of this for a long time beginning when I started in this field in the early 2000s,” he said.