Whenever a major storm hits the American West, pilots are likely to fly into the eyes, clouds are seeded with a substance called silver iodide. The goal is to increase rainfall.
Cloud seeding has been practiced since the 1940s. It has become widespread of late as the West grapples with a drought of historic proportions. States, utility companies and even ski resorts are footing the bill.
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While it was believed for decades to be effective, recent research have helped prove that cloud seeding works, and there is no evidence that silver iodide is harmful at current levels. Experts say that cloud seeding usually increases rainfall by 5% to 15%.
It’s no cure for drought, but cloud seeding can be an important water management tool.
“We can’t make a storm and we can’t create conditions in this storm that are ideal. They happen naturally,” said Jason Carkett, a utility analyst and hydrologist with the Turlock Irrigation District in central California. Turlock started his cloud-seeding program in the 1990s.
“What we’re doing is just taking advantage of the existing conditions, naturally occurring conditions, and trying to make storm again more efficient from a water supply standpoint,” Carkett said.
how cloud seeding works
When done aerially, cloud seeding involves loading an aircraft with silver iodide. Flares are placed on the wings and fuselage.
The pilot reaches a certain altitude, where temperatures are ideal, and flares up in the clouds. Silver iodide causes individual water droplets within clouds to freeze together, forming snowflakes that eventually become so heavy that they collapse.
In the absence of the freezing process, the droplets would not be able to bond together and become large enough to precipitate as rain or snow.
“Clouds are all water in the beginning,” said Bruce Bow, vice president of meteorology at Weather Modification International, a private company that has provided cloud-seeding services since 1961. 50% snow or maybe more. But even if it is, there’s still a lot of liquid water left in there.”
Boe said there is a “window of opportunity” for precipitation to become large enough to “fall on the mountain before it begins to descend further down and thus warm.”
Pilot Joel Zimmer, who works for Weather Modification International, affixes silver iodide flares to the underside of a cloud seeding plane.
Katie Brigham | cnbc
For cloud-seeding pilots like Joel Zimmer, who works with Weather Modification International to seed clouds for the Turlock Irrigation District, flying into a storm can be an exhilarating but intense experience.
“By the time the wheels are up, you’re in a cloud,” said Zimmer, whose route includes seeding over the Sierra Nevada mountains. “And we keep the whole mission in the cloud until we’re shooting back in an airport and then we’re out of the clouds and there’s a scene on the runway. It’s like being a deputy commander in the Navy.” You don’t see anything.”
From a water supply point of view, it is most valuable to seed clouds on mountains where the water is essentially stored as snow until spring runoff.
“When it’s in plains like North Dakota, it’s still a benefit because it helps recharge soil moisture,” Bo said. “But it cannot be stored and used for a later date.”
While Texas uses cloud seeding to help irrigate fields for farmers, it is more common in the West, where states such as Idaho, California, Colorado, Utah, and Wyoming use it to help fill their rivers and reservoirs. Let’s use Most programs use aircraft for cloud seeding, but some use ground-based flares.
“It’s more common than people think,” Carkett said. “More basins have a seeding program than don’t have a seeding program.”
cost and impact
Boe says the cost is almost always worth it.
“It makes a lot of sense for water managers to step forward and do it, even if the increase is on the order of a few percentage points,” he said.
Idaho Power spends about $4 million a year on its cloud-seeding program, which in some areas increases the snowpack by 11% or 12%, resulting in billions of additional gallons of water at a cost of about $3.50 per acre-foot . This compares to $20 per acre-foot for other methods of accessing water, such as through a water supply bank.
And although Turlock sees only a 3% to 5% increase in runoff from his program—which has a maximum budget of $475,000—California will take all the extra water it can get.
“That’s one of the things that makes it so difficult to evaluate, is you don’t see precipitation doubling or tripling,” Boe said. “You see an incremental increase, but you add it during the winter and then it can be significant.”