The large metal bunker with a chimney on top goes mostly unnoticed by skiers zipping by. It’s not a snow-making machine, like those the ski areas are relying on this winter. The chimney releases tiny particles of silver iodide – the seeds that rise thousands of feet into the clouds.

“Water needs some sort of substance to condense upon,” says Tilley. Clouds are made of millions of tiny water droplets, but the droplets don’t automatically fall as rain or snow. They stick to tiny particles like dust.

I think for the entire Intermountain West, it’s becoming more important

If a cloud doesn’t have enough dust, “you have these very static, dead clouds that don’t precipitate, don’t produce any water and just keep moving right through,” he says.

That’s where the silver iodide comes in. Tilley says it’s the right size and shape to help snowflakes form. Cloud seeding only works in certain conditions: cold temperatures with the right wind direction and cloud types. But over a season, Tilley says it can make a difference.

“What we find is a range of anywhere between eight and 15 percent increase in water,” he says.

The silver iodide eventually ends up in the local environment, where some worry it’s a contaminant, though Tilley says tests show it’s only a trace amount.

Cloud seeding has been used for six decades in California. In the early days, it was closer to “magical thinking,” an idea Tilley says has stuck around. “We get voodoo,” he says. “We get Dr. Frankenstein. We get all sorts of things. But we’ve been able to refine the technology.”

No Silver Bullet

“For a long time there’s been hope that we could somehow figure out of a way to squeeze more water out of nature,” says Peter Gleick, president of The Pacific Institute, a water policy think tank.

Gleick says the problem with cloud seeding is that it’s tough to measure or verify how much water it produces and if it’s worth the investment. A review by the National Academy of Sciences in 2003 found that more research needs to be done to prove its effectiveness.

“But even more importantly, it’s limited no matter what,” says Gleick. “We get a certain number of clouds with moisture in them. If we can wring a little more out of those clouds, that’s sort of the idea behind cloud seeding. But we’re not going to wring a lot more out of those clouds.

"So it’s not a silver bullet," he says. "There is no silver bullet for California’s water problems.”

Nine other western states also use cloud seeding, where it’s commonly done with airplanes. The Desert Research Institute is also looking into using drones, potentially cutting the cost of flights.

Across California, water agencies and utilities spend $3-to-5 million a year on seeding, which is estimated to boost runoff by around four percent. That might not sound like much, but these days when every drop counts, Jeff Tilley says cloud seeding getting a second wind.

“I think for the entire Intermountain West, it’s becoming more important,” he says. “It’s not going to be the whole answer but it can be one tool in the toolkit and it’s a cost-effective one.”

Snowstorms from Dust Storms

Improving cloud seeding may depend on scientists unraveling something that’s still mysterious: what exactly makes it rain?

“It’s incredibly complicated,” says Kim Prather, who studies atmospheric chemistry at the University of California-San Diego.

Prather wanted to know why some clouds produce snow in the Sierra Nevada and others don’t. So, she and her team flew an airplane through the clouds, testing them to see what kinds of tiny particles were forming snowflakes.

What she found was a big surprise. On snowy days, the clouds contained dust from a faraway source.

“Dust had made its way across the Pacific, clear from Asia and even Africa, the Middle East where there are these big dust storms,” she says. “Takes about 7-to-10 days to get here, but it makes it. It’s not a lot of dust. It’s just the right amount of dust that seeds the very top of the clouds.”

Prather says that type of dust can boost snowfall, but other kinds of particles seem to have the opposite effect. Air pollution, from California sources and all the way from Asia, could be adding too many tiny cloud seeds.

“There’s only so much water available and in order to get rain, you have to have big enough droplets for them to fall. The more seeds you have, you have many more tiny droplets. If you have too many seeds, you’re not going to get precipitation out of that cloud.”

“Potentially it’s us affecting our own water supply,” she says. “Potentially it’s stuff coming from much farther away and to be able to sort that out, we’re just at the tip of the iceberg.”