How cloud seeding can help replenish reservoirs in the West
Whenever there’s a big storm in the western United States, pilots are likely to fly into the eye, seeding clouds with a substance called silver iodide. The goal is to increase rainfall.
Cloud seeding has been around since the 1940s. It has become more widespread of late as the West battles a drought of historic proportions. States, utility companies, and even ski resorts are footing the bill.
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Although it was assumed for decades that it would be effective, recent studies have 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% to 15% increase in precipitation.
It’s not a cure for drought, but cloud seeding can be an important water management tool.
“We can’t make a storm happen and we can’t create conditions in this storm that are ideal. That happens naturally,” said Jason Carkeet, utility analyst and hydrologist with the Turlock Irrigation District in central California. Turlock began its cloud seeding program in 1990.
“What we’re doing is just taking advantage of existing conditions, natural conditions and trying to make the storm again more efficient from a water supply perspective,” Carkeet said.
How cloud seeding works
When done from the air, cloud seeding involves loading an aircraft with silver iodide. Flares are attached to the wings and fuselage.
The pilot reaches a certain altitude, where the temperatures are ideal, and shoots the flares into the cloud. Silver iodide causes individual water droplets within clouds to freeze together, forming snowflakes that eventually become so heavy that they fall.
In the absence of the freezing process, the droplets would not stick together and would become large enough to precipitate out as rain or snow.
“The cloud initially is all water,” said Bruce Boe, vice president of meteorology for Weather Modification International, a private company that has provided cloud seeding services since 1961. “Eventually, as you get closer to the top of the mountain, you can be 50% ice or maybe more than that. But even if it is, there’s still a lot of liquid water left.”
Boe said there is a “window of opportunity” for the precipitation to be large enough to fall “before it gets to the top of the mountain and starts going downhill and therefore warms up.”
Pilot Joel Zimmer, who works for Weather Modification International, places silver iodide flares on the underside of a cloud seeding plane.
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For cloud seeding pilots like Joel Zimmer, who works with Weather Modification International to seed clouds for the Turlock Irrigation District, flying into the storm can be an exhilarating but intense experience.
“By the time the wheels are up, you’re in the clouds,” said Zimmer, whose route involves planting over the Sierra Nevada mountains. “And we’re in the cloud for the whole mission until we shoot an approach back to an airport and then we come out of the clouds and have a picture of the runway. It feels like you’re a deputy commander in the Navy. You don’t see anything.” .
From a water supply perspective, it is more valuable to seed clouds over mountains, where the water is essentially stored as snow until spring runoff.
“When you’re out on the plains like North Dakota, it’s still a benefit because it helps recharge soil moisture,” Boe said. “But it can’t be stored and used for a later date.”
While Texas uses cloud seeding to help irrigate farmers’ fields, it’s more common in the west, where states like Idaho, California, Colorado, Utah and Wyoming use it to help fill their rivers and reservoirs. Most programs use aircraft for cloud seeding, but some use ground-based flares.
“It’s much more common than people think,” Carkeet said. “More basins have a planting program than those without a planting program.”
costs and impact
Boe says the cost is almost always worth it.
“It makes a lot of sense for water managers to go ahead 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 produces an 11-12% increase in snowpack in some areas, resulting in billions of gallons of additional water at a cost about $3.50 per acre-foot. That compares to about $20 per acre-foot for other methods of accessing water, such as through a water supply bank.
And while Turlock only sees a 3% to 5% increase in runoff from its program, which has a maximum budget of $475,000, California will take all the extra water it can get.
“It’s one of the things that makes it so hard to gauge is that you don’t see precipitation that doubles or triples,” Boe said. “You see an incremental increase, but you add it up over the course of a winter and then it can be significant.”
Watch the video to learn more about what it takes to make it rain.