How countries across the West use cloud seeding to make it rain

Whenever there’s a big storm in the American West, pilots are likely to fly in the eye, seeding the clouds with a substance called silver iodide. The goal is to increase precipitation.

Cloud seeding has been around since the 1940’s. It has become rife recently as the West grapples with a drought of historic proportions. States, utility companies, and even ski resorts foot the bill.

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While for decades it was assumed to 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 generally leads to 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 create a storm, we can’t create ideal conditions in this storm. It happens naturally,” said Jason Carkett, a utility analyst and hydrologist at the Turlock Irrigation District in central California. Turlock started the cloud seeding program in 1990.

“What we’re doing is just taking advantage of existing conditions, naturally occurring conditions, and trying to make the storm again more efficient from a water supply perspective,” Carkett said.

When done from the air, cloud seeding involves loading the plane with silver iodide. Flares are placed on the wings and fuselage.

The pilot gets to a certain altitude, where the temperatures are ideal, and shoots 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 will not bind together and become large enough to precipitate as rain or snow.

“The cloud at first is all water,” said Bruce Boe, vice president of meteorology at Weather Modification International, a private company that has provided cloud seeding services since 1961. It contains 50% ice or maybe more. But even if it was, there would still be a lot of liquid water left.”

There’s “a window of opportunity” to get big enough rain to come down “before it hits the mountaintops and starts to get so warm,” Poe said.

For cloud seeding pilots like Joel Zimmer, who works with Weather Modification International to seed clouds for the Turlock Irrigation Area, flying into a storm can be an exhilarating yet intense experience.

“By the time the wheels are up, they’re in a cloud,” said Zimmer, whose route includes planting seeds atop the Sierra Nevada mountains. “And we’re in the cloud the whole mission until we shoot all the way back to the airport and then we get out of the cloud and we get a visual on the runway. It’s like you’re a Navy subcommander. You don’t see anything.”

From a water supply perspective, it is best to plant clouds over mountains, where water is stored mainly as snow until runoff in the spring.

“When you’re on the plains like North Dakota, it’s still a benefit because it helps recharge soil moisture,” Bo said. “But it cannot be stored and used at a later time.”

While cloud seeding is used by Texas to help irrigate fields for farmers, it is more popular in the West, where states such as Idaho, California, Colorado, Utah, and Wyoming use it to help fill rivers and reservoirs. Most programs use aircraft to seed clouds, but some use ground-based flares.

“It’s more common than people think,” Carkett said. “More ponds have a seeding program than no seeding program.”

Boe says the cost is almost always worth it.

“It makes a lot of sense for water managers to go ahead and do this, even if the increase is on the order of a few percentage points,” he said.

Idaho Power spends about $4 million annually on its cloud seeding program, which results in an 11% or 12% increase in snow mass in some areas, resulting in billions of gallons of additional water at a cost of about $3.50 per acre foot. That compares to about $20 per acre per foot for other methods of water access, such as a water supply bank.

And although Turlock only sees a 3% to 5% increase in runoff from its program—which has a maximum budget of $475,000—California will consume all the extra water it can get its hands on.

“One of the things that makes it difficult to assess is, don’t you see a doubling or tripling of rainfall,” Bo said. “You see a gradual increase, but you add it over the course of the winter and then it can be significant.”

Watch the video to learn more about precipitation requirements.