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They say that everyone complains and yet no one ever does anything about the weather. 

But Colorado is actually trying very, very hard to do something, possibly a quite large something: Expanding decades of cloud seeding to an eighth campaign to combat the 22-year drought by wringing more snow from every storm tantalizing the biggest river basins. 

The next time promising snow clouds gather over the St. Vrain basin west of Longmont, newly placed silver iodide guns will shoot the chemical high into the gloomy skies in the hope of coaxing an extra 10% to 15% of snowpack from the atmosphere. 

The new St. Vrain Creek effort joins existing blasts of silver iodide targeting heavy snow clouds for decades above the upper Colorado River, the North Platte, the Gunnison, and Grand Mesa. Another study is underway to see if cloud seeding could boost flows in the Yampa and White River basins in northwestern Colorado. 

The high-tech rain dances are paid for by an intriguing combination of thirsty customers. State governments want to boost water resources and tourism. Ski areas seek the deepest possible powder. And downhill states like Arizona and California pay up as they choke on new federal restrictions on Colorado River water use, with climate change quickly altering reality for 40 million snowpack-dependent Westerners. 

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Colorado spends millions of dollars on the ground- and aerial-based cloud seeding programs because it works, state water officials say. 

“We typically like to say, on an average storm, we can increase it 8% to 12% of the snow-water equivalent,” said Andrew Rickert, weather modification program manager for the Colorado Water Conservation Board. 

“If we have a storm coming through that’s going to give us 10 inches, we can get another inch out of that storm,” Rickert said. “But over the duration of an entire winter over hundreds of thousands of square miles, we can add hundreds of thousands of acre-feet of water to spring runoff.” For perspective, federal officials have warned the seven Colorado River Basin states need to find 2 million to 4 million acre-feet of cuts to water use for 2023 because of drought soaking up historic river flows. 

An island emerges from seasonal low water levels at Dillon Reservoir Thursday morning in Summit County. The Denver Water’s reservoir diverts the water through the Harold D. Roberts Tunnel under the Continental Divide into Colorado’s Front Range. (Hugh Carey, The Colorado Sun)

The St. Vrain and Left Hand Water Conservancy District is enthusiastic to be the latest Colorado region to come on board with snow-boosting science. Their legal “Notice of Intent to Modify the Weather” hit local newspapers in September, and said the cloud seeding will begin this month above 6,500 feet. The new territory could send silver iodide shooters anywhere from Larimer County to Park County and points in between. 

The St. Vrain district doesn’t deliver water to consumers, but its mission is to enhance water resources for all users in the area that the creek drains, from Longmont and Lyons water agencies to Front Range farmers to snow recreation in Boulder and Larimer counties. In developing a new long-term water plan, St. Vrain basin residents made it clear they did not want massive new storage in the form of a dam and reservoir on the relatively free-flowing creek. 

Long established cloud seeding programs provide the kind of “holistic, sensible approach” to water supply that can “do it in a way that marries up with the values of our community,” district Executive Director Sean Cronin said. 

North American Weather Consultants technician Parker Cammans installs a cloud seeding device near Williams Fork Reservoir, Nov. 3, 2022, outside Kremmling. Cloud seeding, a weather modification tool, releases silver iodide and other particles to coax additional precipitation from promising clouds. (Hugh Carey, The Colorado Sun)
Gauges for the cloud seeding device near Williams Fork Reservoir, Nov. 3, 2022, outside Kremmling.

Those community values included a clear look at the environmental science of cloud seeding, and any potential dangers of introducing silver iodide into the atmosphere above the St. Vrain. When shot or dropped into clouds full of promising moisture, tiny silver iodide particles form a nucleus that encourages droplets to condense and become snowflakes. 

Environmental groups have not raised serious objections to the cloud seeding programs on contamination issues, experts say. The silver emitted in cloud seeding barely registers as trace amounts. 

“If you take silver iodide by the spoonful, yes, it’s not very good for you,” said William Cotton, professor emeritus of meteorology at Colorado State University. “But in the amounts on the ground in snowpack, it takes extremely advanced instrumentation to even detect it.”

That doesn’t mean everyone agrees on whether it works. “Notice to Modify Weather?” Some don’t notice. 

But cloud seeding has not been proven effective in boosting the available water supply, said nonprofit Western Resource Advocates water policy analyst John Berggren. 

“Some of the studies we’re aware of suggest cloud seeding may be able to increase precipitation 5% to 15%, but it’s unclear how that translates into streamflow and water availability,” Berggren said. “We believe there are numerous other proven and legal ways for communities to increase water security, namely conservation and demand management.”

CSU’s Cotton isn’t even so sure about those studies showing “5% to 15%” enhancement. The “sometimes desperate” search for water in the West is understandable, but “might not be as promising as people wish” in delivering extra snowpack, Cotton has written for science websites

North American Weather Consultants technician Parker Cammans installs a cloud seeding device near Williams Fork Reservoir, Nov. 3, 2022, outside Kremmling. (Hugh Carey, The Colorado Sun)

One early cloud seeding study that CSU participated in, often cited by weather modification proponents, claimed to find 10% to 15% enhancement when done well, including shooting silver iodide from the highest possible altitudes and finding the best wind and moisture conditions, Cotton said. 

“So that really excited people and everything,” Cotton said. 

But more recent studies using specialized cloud radars and aircraft sampling, in Wyoming and Idaho snow basins, show “more like 1% to 2% increases in precipitation,” he said. 

“The amounts still are really questionable. A few percent makes it really hard to be cost effective. Five to 10% would really be more cost effective,” Cotton said. The method that appears to be most effective, spraying silver iodide into the clouds from airplanes, is also the most expensive. 

Cloud seeding can’t create storms where only drought skies exist. To be most effective, silver iodide has to be shot into heavy clouds loaded and ready to go with moisture, that just need tiny dust or ice particles to form good snow.

Signs of low water levels of Williams Fork Reservoir near Kremmling in early November. Clouds are formed by evaporation from liquid water into the atmosphere before freezing around dust and other particles. Cloud seeding gives the existing moisture more particles to form heavy snowflakes, promising an extra percentage from each storm as the flakes fall back to earth. (Hugh Carey, The Colorado Sun)

Only about a dozen storms during a winter season meet the criteria for good cloud seeding results, experts say. 

A 2005 study the state water conservation board posts on its website describing weather modification concludes that year’s model produced “very small differences between seed and control precipitation predicted by the model,” and that the results were “very disappointing and not expected at the onset of this project.” 

All but one of Colorado’s cloud seeding programs use ground-based devices to hit the clouds from below. Jackson County, touching the Wyoming border, has a dual aerial and ground program to try to add water to the North Platte River.

The seven previously existing programs cost about $1 million a year to run statewide, according to Denver Water, which is one of about 40 organizations that help fund the seeding. A coalition of Front Range consumer water providers, from Denver to Aurora to Pueblo to Northern Water, have sponsored cloud seeding for the Central Colorado Mountains River Basin program, though not every water agency joins every year.

The central program covers four remote control generators at high elevations, and over 20 manually operated seeding machines, Denver Water said. The seeding targets clouds passing over Eagle, Grand, Pitkin and Summit counties. The Front Range water providers own rights to water on the west side of the Continental Divide in the Colorado River basin, and bring it through the divide into the Arkansas and South Platte river systems for consumer use. 

St. Vrain is calling its initial entry a pilot program. The district will pay about $50,000 for operations, while the state covers $86,000 for the generators and installation, Cronin said. 

Cronin and his engineers believe the most recent studies offer good proof of the concept. The St. Vrain basin generates about 80,000 acre-feet of water a year, Cronin said, meaning a 3% to 10% enhancement could bring up to 8,000 additional acre-feet. An acre-foot can serve the water needs of about two typical households for a year, according to most estimates. 

Sun shines on mountains in Eagles Nest Wilderness before a snowstorm in early November near Silverthorne. (Hugh Carey, The Colorado Sun)

The $136,000 cost is a fair investment in search of a “rising tide that lifts all boats” of basin water users, Cronin said.

But does cloud seeding here steal water from where it would otherwise fall, over there? 

That’s always the second question after “Does it work?”, cloud seeding proponents say. Western Resource Advocates raises issues about whether it does in fact do that, rob Peter to pay Paul’s water bills. 


“If cloud seeding is used ‘upstream’ of another basin with senior water rights, would that be in violation of prior appropriation? Could the ‘downstream’ basin sue or ask for a compact call? If you start quantifying the savings from cloud seeding, you could run into thorny legal issues,” Berggren said. 

Those issues have not come up, according to Rickert and other seeding proponents. The programs are in fact sponsored by water users “over there,” in the form of California, Nevada and Arizona water officials helping to pay for cloud seeding where their supply is largely stored, in the Colorado high country snowpack. 

St. Vrain engineer Scott Griebling said a typical storm drops less than 10% of the water available in that set of clouds. If cloud seeding manages to boost that small number by 10%, that’s only taking 1% more of the total water. 

“There’s still plenty of water left in the cloud,” he said.

For true water geeks, and in Colorado there are plenty, the next question is inevitably, “Who owns the new water?” 

The answer is imprecise, but comes down to who really “owns” the entire state water pool. 

St. Vrain calls statewide cloud seeding “raising the dam” for all of Colorado, backing up new water through a virtual reservoir. Any “new” water from cloud seeding accrues to the benefit of water rights holders from that basin who, because of drought or holding a more junior appropriation, have lately been right on the cusp of losing their full share. 

Flames ignite on a cloud seeding device as it releases hydrogen and other chemical solutions near Williams Fork Reservoir, Nov. 3, 2022, outside Kremmling. (Hugh Carey, The Colorado Sun)

The additional water also makes it easier for Colorado and various conservancy districts to meet compact calls from other states, like Nebraska demanding its share from the South Platte, or Nevada policing uses on the Colorado River.

“This is the only way to physically add water to a basin,” Rickert said. “Whoever has the senior water rights is still going to be able to pull their water, but we’re just trying to add more to the system as a whole.”

Michael Booth is the Sun’s environment writer, and co-author of the Sun’s weekly climate and health newsletter The Temperature. He is co-author with Jennifer Brown of the Colorado Book Award-winning food safety investigation “Eating Dangerously.” Booth was part of teams that won two Pulitzer Prizes for breaking news. He also writes frequently about inexplicable obsessions that include tamarisk, black-footed ferrets and tire fires. Booth also serves as the underpaid driver for four children, and plans to eventually hike every inch of Colorado.