Cloud seeding bolsters Sierra snow
Lake Tahoe is well-known as a winter recreation paradise, outdoors enthusiasts taking advantage of its ample winter snowfall.
But few who take advantage of the bountiful snowpack realize that researchers have been artificially boosting the Sierra Nevada’s snowfall for more than half a century.
Cloud seeding, as the weather-modification process is known, is conducted in the Sierra Nevada mountain range primarily to increase municipal water supplies in Nevada, said Operations Manager Tom Swafford with the Desert Research Institute’s Cloud Seeding Program.
“The end goal is to increase the snowpack; I mean we live in a desert so we need more snow than we have,” said Swafford, his words punctuated by puffs of frozen breath as he hiked through a foot of fresh snow to an Institute cloud-seeding site near Donner Summit. “Specifically we are tasked with increasing the snowpack in the mountain ranges that drain into the state of Nevada, so that would be the Sierra into the Truckee River, and further south in the Sierra, the Carson and Walker rivers.”
The 5-foot tall metal box, nestled in a bed of fresh Sierra snow, a quarter mile from the Old U.S. 40 highway, houses sophisticated computer equipment, propane fuel and 135 gallons of a liquid silver iodide mixture, the raw material for making snow. When burned, the tiny silver iodide crystals in the smoke rise into the clouds, and serve as a nucleus for snowflakes to grow on.
The Donner Summit station is one of five cloud-seeding generators the Reno-based research institute operates in the Lake Tahoe-Truckee area. The Institute operates 22 other cloud-seeding generators located near the Ruby, Tuscarora and Toiyabe mountains.
Most are equipped with a 15-foot tower with a burner unit, a giant metal cylinder, on top. That’s where the technicians burn the rain-making solution to create a two-mile long cloud consisting of billions of microscopic particles, which mixes with the storm clouds overhead.
Managers like Swafford can activate the generator remotely through an Internet connection from their office in Reno. The system remains on through the storm cycle and can significantly increase the snowpack.
“This plume can be 10 to 12 to 15 miles long, and up to one to two miles wide. You’re talking 20 square miles, and that’s one generator,” Swafford said. “That’s a huge amount of water. If you get an inch over a 20-square-mile area, that’s hundreds of acre-feet if not thousands” of acre-feet of water.
He said the annual budget to operate the silver-iodide generators for 450 hours is $500,000. The money comes from the state of Nevada.
The history of cloud seeding
Serious scientific research of weather modification and cloud seeding began in 1946. The two key individuals involved were Irving Langmuir and Vincent Schaefer.
Schaefer first discovered he could make fog freeze by breathing into a freezer box and introducing dry ice into the “cloud.” This happens because the dry ice chills the cloud to about -40 degrees Centigrade, the temperature at which water drops will freeze without the aid of a small impurity, or nucleus.
Later in 1946, Bernard Vonnegut, the late author Kurt Vonnegut’s brother, was working on nucleation processes at the General Electric labs in Schenectady, N.Y. He and Schaefer found that silver iodide could produce ice particles in a freezer box almost as quickly as dry ice.
Cooling to -40 C wasn’t required because the silver iodide impurity acted as the nucleus that the ice crystal formed on. Those initial experiments then led to experiments in actual clouds with both dry ice and silver iodide.
Researchers began cloud seeding in winter clouds over the Sierra Nevada in the early 1950s.
The seeding agent is a mix of acetone and silver iodide. It sounds scary, but when burned, the acetone breaks down into hydrogen and water, two naturally occurring substances, said Tom Swafford of the Desert Research Institute. Once the acetone is burned off, it leaves billions of super-heated silver iodide particles, or ice-condensation nuclei. As those particles swirl in the clouds, they start running into tiny droplets of water and growing a snow crystal. The process replicates the natural process of a snowflake growing around a microscopic piece of dust, sand or salt, Swafford said. Silver iodide is highly insoluble in water and has a crystalline structure similar to that of ice.
With numerous cloud seeding projects in the Sierra Nevada, the environmental concerns have been addressed in several environmental impact studies. Officials at the Desert Research Institute used the findings of a Bureau of Reclamation Environmental Assessment that was done for a cloud seeding project in the American River Basin. The finding of no significant impact covered not only silver toxicity, but also the impact of additional snow and runoff on vegetation, buildings and highway safety.
— Information courtesy of Arlen W. Huggins Associate Research Scientist of the Desert Research Institute in Reno.
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