You can’t make something out of nothing.
But you can use a little bit of silver iodide to make a lot more snow.
Idaho Power injects silver iodide into clouds to coax more snow out of winter storms, a process called cloud seeding.
“Most winter storms are inefficient. Most of the water vapor actually just transitions downwind and doesn’t produce snowfall on the ground. So what cloud seeding does is we try to start that process of growing snowflakes sooner and more efficiently,” Derek Blestrud, a senior atmospheric scientist with Idaho Power, said in an interview with the Idaho Statesman.
More snow means more water in Idaho’s reservoirs and more water running through hydroelectric dams. According to Idaho Power, the additional runoff caused by cloud seeding in Idaho energizes 57,5000 homes annually.
Cloud seeding has been around for decades. Scientists working for General Electric discovered in 1946 that injecting silver iodide could make pre-existing clouds produce more rain. But myths still persist about how cloud seeding works and the safety of the process.
So how does cloud seeding work?
Cloud seeding might seem like magic, but there’s chemistry and physics behind the scenes.
Clouds contain super-cooled liquid that is cold enough to form snow, but there aren’t enough particles to start snowflake formation.
“We add silver iodide to teach the cloud how to form snow,” Blestrud said. “For a snowflake to form, crystals have to form around something like dust or ice. We add silver iodide to add nuclei for snowflakes to form. Silver iodide is hexagonal in shape so it works very well to start precipitation in a cloud.”
Idaho Power uses two injection methods to release silver iodide into the clouds: ground-based towers and a modified aircraft. To release from the ground, Idaho Power injects a silver iodide solution into a small fire burning inside the ground-based tower, and the silver iodide is suspended in the smoke as it floats into the atmosphere. To release from the aircraft, the pilot presses a button on a modified plane with flares on the wings, lighting the flares and releasing silver iodide in the flare smoke.
“Twenty to thirty minutes from silver iodide injection, snowflakes fall to the ground. This timing is similar to the timing of natural snowflake formation,” Blestrud said.
Idaho Power currently cloud-seeds the mountain ranges surrounding the Payette, Boise, Wood, Henry’s Fork and Upper Snake rivers November through April.
Is cloud seeding harmful?
Although cloud seeding has been used for decades, the public remains skeptical.
“It is a common misconception that silver iodide is harmful. Silver iodide occurs naturally and cloud seeding injects amounts smaller than we can detect,” said Sarah Tessendorf, a cloud physicist at the National Center for Atmospheric Research. “Snow and water sampling shows that silver from cloud seeding is well below other sources like natural mineral dusts and very far below EPA regulations.”
Silver iodide is a chemical compound, yellow in color and made from silver and iodide bonded together.
“This silver iodide bond is a very strong bond. It is insoluble in water. It takes a lot of external energy or heat to break apart that bond, and we typically do not find that strong energy in nature,” said Shaun Parkinson, meteorology and cloud seeding leader with Idaho Power. “Silver iodide is inert, meaning it does not bond to any of the biological tissues from humans to fish to macroinvertebrates.”
To test for silver iodide in snow and water, scientists run samples through a mass spectrometer, a machine that separates individual elements from the sample. According to Parkinson, silver can be detected in the levels of 4 to 40 parts per trillion in the cloud-seeded snow; for comparison, that is 1,000 times lower than silver found in the saliva of someone with a silver tooth filling.
How effective is cloud seeding?
According to Tessendorf, cloud seeding can increase winter storm efficiency by 5 to 15 percent. But there is not scientific consensus on its efficiency.
A 2010 reassessment of 1970’s Israeli cloud seeding, published in Atmospheric Research, concluded that increased rain was the result of unusual natural rain patterns and not cloud seeding, according to a Mother Nature Network article in 2018.
It has been difficult to measure the success of cloud seeding, according to Tessendorf, because once a cloud is seeded, scientists cannot know how much precipitation the cloud might have dropped without human-influence.
That same MNN article also cited the Wyoming Weather Modification Pilot project, which ran for six years in two adjacent mountain ranges in southern Wyoming, and concluded in 2013. The researchers on the project observed a 5-15 percent increase in snowfall. But the project executive summary reported that this increase might not be accurate because of unintended drifting of silver iodide onto unseeded mountain ranges and insufficient quantities of silver iodide reaching the intended mountain range target.
Here in Idaho, Idaho Power says that average cumulative precipitation increases 4.9% on Henry’s Fork and 15.1% on the Boise River basin because of cloud seeding, with the other rivers falling in between.
“So to help put this volume of water in context, we have about a million additional acre-feet annually. That energizes 57,500 homes in Idaho,” Parkinson said.
This additional water from cloud seeding is intended to help Idaho Power keep pace with growth and reduce dependency on fossil fuels.
“If this were to displace or offset the use of coal, that’s 736,000 tons of fuel, or about 380,000 tons of coal,” Parkinson said.
Idaho Power plans to operate on 100 percent clean energy by 2045.
Does cloud seeding ‘steal’ water from people downwind?
“The atmosphere is very dynamic. There is always a pipeline of moisture coming in. So it replenishes that little extra water we take out quite rapidly,” Parkinson said.
Cloud seeding, whether from the ground or the air, targets a specific area for more precipitation. But silver iodide can remain in the storm system as it moves downwind.
“Studies show there can be additional positive effects even up to 250 kilometers (155 miles) downwind of the target base. And past 250 kilometers the effects are positive or neutral, so more rain or no difference in rain,” Parkinson said.
Although cloud seeding has been around for decades, it is still an active research area.
“The effects of cloud seeding are hard to quantify. We are using new models to evaluate the impact of cloud seeding,” Tessendorf said. “We have observed direct evidence of the pattern of snow produced by cloud seeding. But we still have more questions.”
The future of cloud seeding in an uncertain future
The climate is changing. Shifting precipitation patterns will bring floods and droughts to the West, including Idaho, according to scientists.
“We cannot use cloud seeding to fix a drought,” Blestrud said. “We can add snowpack to equip our reservoirs for a drought and build supply for when we need it. But without clouds we cannot produce more precipitation.”
According to Tessendorf, the impact of additional human-caused greenhouse gases in the atmosphere changes when and where clouds form. With warmer temperatures, clouds can disappear faster and form rain instead of snow or not form precipitation at all.
“Humans have an impact on the atmosphere every day. We are already affecting clouds and precipitation because human-caused pollutants are intricate to the water cycle and can change water patterns,” Tessendorf said. “With cloud seeding we are mitigating our impact. The state of the science is exciting. Stay tuned.”
Rachel Hager is writing for the Idaho Statesman this summer on a fellowship through the American Association for the Advancement of Science. She is a master’s student in ecology at Utah State University and earned a bachelor’s degree in biology at Bryn Mawr College.