Whenever a major storm rages in the American West, pilots are likely to fly into the eyes, seeding the clouds with a substance called silver iodide. The goal is to increase rainfall.
Cloud seeding has been around since the 1940s. It has become widespread in recent times as the West grapples with a drought of historic proportions. States, utilities and even ski resorts pay the bills.
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Although it has been believed for decades to be effective, recent research helped prove that cloud seeding works, and there is no evidence that silver iodide is harmful at current levels. Experts say cloud seeding typically results in a 5-15% increase in rainfall.
It is not a cure for drought, but cloud seeding can be an important water management tool.
“We cannot create a storm and we cannot create ideal conditions during this storm. It happens naturally,” said Jason Karkit, a utility analyst and hydrologist from Turlock Irrigation County in central California. Turlock launched its cloud seeding program in 1990.
“What we are doing is just taking advantage of existing conditions, natural conditions, and trying to make the storm more efficient again in terms of water supply,” Karkit said.
How cloud filling works
When performed from the air, cloud seeding involves loading the aircraft with silver iodide. Rocket launchers are located on the wings and fuselage.
The pilot reaches a certain altitude where the temperature is ideal and launches rockets into the cloud. Silver iodide causes individual water droplets in clouds to freeze, forming snowflakes that eventually become so heavy that they fall.
In the absence of the freezing process, the droplets would not coalesce and would become large enough to fall as rain or snow.
“A cloud is originally made up of water,” said Bruce Boe, vice president of meteorology at Weather Modification International, a privately held company that has been providing cloud seeding services since 1961. be 50% ice or maybe more. But even if that’s the case, there’s still a lot of liquid water left.”
Bo said there is a “window of opportunity” to get heavy enough rainfall to fall “before they reach the top of the mountain and start descending and thus warm up.”
Pilot Joel Zimmer of Weather Modification International attaches silver iodide flares to the underside of a cloud seeding plane.
Kathy Brigham | CNBC
For cloud seeding pilots like Joel Zimmer, who works with Weather Modification International on cloud seeding for the Turlock Irrigation District, flying into a storm can be an exhilarating but stressful experience.
“By the time the wheels are up, you’re in the clouds,” said Zimmer, whose route includes seeding over the Sierra Nevada mountains. “And we are in the cloud for the entire mission until we film the approach to the airport, and then we jump out of the clouds and see the runway. I don’t see anything.”
From the point of view of water supply, it is most valuable to sow clouds over mountains, where water is stored in the form of snow until spring runoff.
“When it’s in the plains like North Dakota, it’s still useful because it helps restore soil moisture,” Bo said. “But it cannot be saved and used at a later date.”
While in Texas cloud seeding is used to irrigate fields by farmers, it is more common in the West, where states like Idaho, California, Colorado, Utah, and Wyoming use it to fill their rivers and reservoirs. Most cloud seeding programs use aircraft, but some use ground-based rockets.
“It’s much more common than people think,” Karkit said. “More pools have a seeding program than do not have a seeding program.”
Bo says the cost is almost always worth it.
“It makes sense for water managers to go ahead and do this, even if the increase is in the order of a few percentage points,” he said.
Idaho Power spends about $4 million a year on its cloud seeding program, which results in an 11–12% increase in snow cover in some areas, resulting in additional billions of gallons of water at a cost of about $3.50 per acre foot. This compares to about $20 per acre-foot for other ways of accessing water, such as water supplies.
And while Turlock sees only a 3-5% increase in flow from his $475,000 maximum budget program, California will take all the extra water it can get.
“One of the things that makes it so difficult to estimate is that you don’t see a doubling or tripling of rainfall,” Bo said. “You see a gradual increase, but if you add it over the winter it can be significant.”