Coercing the clouds

As drought worsens and snowpack dwindles, cloud seeding expands east of the Continental Divide for the first time in Colorado

A remote cloud seeding generator used by the North American Weather Consultants. Courtesy NAWC.

Colorado is turning to cloud seeding to beef up snowpack and meet growing water needs, including a new pilot project here in Boulder County.

The project, spearheaded by the St. Vrain and Left Hand Water Conservancy District, will launch this winter west of Longmont along the St. Vrain Creek, pending state approval of a permit.

Cloud seeding sprays tiny crystals of silver iodide into a storm cloud, increasing the cloud’s ability to produce precipitation by introducing more particles that provide a base for snowflakes to form. 

“Weather modification isn’t like a silver bullet that’s going to solve everything,” says Andrew Rickert, a weather modification program manager at the Colorado Water Conservation Board (CWCB). “But we strongly believe it’s one of the tools in a toolbox that we can use to help our state.”

The state’s weather modification program began operation in 1972, when Vail Resorts received the first cloud-seeding permit. Each permitted program since has been west of the Continental Divide. If approved, the permit for the Boulder County cloud-seeding project will be the first conducted east of the Continental Divide in the state. 

The Water District pursued the permit to address community concerns over water supply. 

“We really felt like the timing, the cost and the opportunities were really well aligned,” says Sean Cronin, executive director at the District. “[We] decided that it would be good to demonstrate some leadership and be a pilot for weather modification on this side of the hill.”

Of the eight cloud-seeding projects in Colorado, seven use ground-based generators to float silver iodide into clouds, while one project uses planes to seed clouds aerially. The Boulder County project’s two generators will be ground-based. 

The expansion of these programs into the Front Range comes on the 100-year anniversary of the November 1922 Colorado River Compact signing — a defining moment in Colorado River Management.  

A remote cloud seeding generator used by Desert Research Institute for the San Juan Mountain Program. Photo by Adam Rickert.

The agreement allocated 7.5 million acre feet of the river to both of the newly established Upper (Colorado, Utah, New Mexico, Wyoming) and Lower (California, Arizona, Nevada) basins. 

Today, scientists recommend water managers plan for the river to provide just 9 million acre feet annually — half of the assumed 18 million acre feet the Colorado Compact allocated. 

Mitigating the spiraling reduction of flow in the Colorado River is one reason cloud seeding programs exist. Rickert says cloud seeding can result in 8-12% of snow water equivalent, a measurement that shows exactly how much water is actually in the snowpack. 

“So if a storm is going to come through and drop 10 inches and we seed that storm, we can get an extra inch (of precipitation) out of the storm,” he says. 

But, those numbers are not a given — they depend on the amount of “seedible” storms that pass over a seeding generator, which can vary. 

Cloud seeding generators need a few specific characteristics for optimal success, including storms between 5 and 23 degrees Fahrenheit and wind above 5 knots (depending on how far the generator is from the target area). Stations also rely on directional wind getting pushed up the side of a mountain, known as an orographic lift, to thrust the silver iodide into the cloud base. 

Rickert says the most important factor is already in the clouds.

“The main thing you need in a storm is supercooled liquid water,” he says, so the ice-crystal-mimicking silver iodide has something to “bind” with. Silver iodide can also produce snowflakes at warmer temperatures, meaning more portions of storm clouds can produce snow.

Rickert guesses CWCB will see somewhere between 24 and 35 suitable storms throughout the season, which lasts from Nov. 1 to April 15. 

Once CWCB spots a suitable storm coming toward a generator, a propane flame ignites and vaporizes the silver iodide solution, sending it up into the clouds.

“We are just kind of nudging that cloud to actually make some of that precipitation fall out as snowpack,” Rickert says. 

The St. Vrain and Left Hand Water Conservancy District’s pilot program will have two remote generators located at the base of the foothills to target upslope conditions from storms from the east. The remote generators will be controlled by North American Weather Consultants, a weather forecast service in Salt Lake City.

Scott Griebling, water resource engineer at the Water District, says the program costs roughly $140,000. The CWCB is funding equipment costs (about $90,000), and the District is covering operational costs (about $50,000). 

The District’s project started with public support. In 2020, the District asked voters to support a tax increase to implement the plan. Cronin says voters approved at nearly 70%.  

“There’s agreement that more water can do good for the valley,” he says.

Despite widespread support, some community members were concerned if cloud seeding in their area would take water from other places down wind.

Rickert says the “robbing Peter to pay Paul argument” is one of the most common concerns he hears across the state, but it’s a misconception.

“There have been so many studies conducted that have shown not only is that false, but if anything, cloud seeding can increase precipitation downwind,” he says. 

Rickert also says the technique doesn’t have adverse environmental consequences. Silver iodide is found naturally in the environment, and is not known to be harmful to humans or wildlife. 

The District will use its permit as a test phase to see if it is worth additional investments. 

“We’re just excited about this possibility and really looking forward to getting a season’s worth of operations completed so we can evaluate how to move forward,” says Griebling. 

Sources interviewed for this story are confident the project permit will be approved in the coming weeks. Once the District has the permit from the state, it plans on installing generators by the end of November. 

“Once the equipment’s installed,” says Cronin, “we begin the snow dance and root for the storms we can successfully seed.”