Why Do Some Clouds Drop Rain, While Others Don't?

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Recent storms in California haven't been enough to save the state from a serious drought and now, the rainy season is winding down. Scientists are trying to understand why some storms unload lots of rain and snow in California and others don't. As Lauren Sommer reports from member station KQED in San Francisco, there could be a link to dust storms thousands of miles away.

LAUREN SOMMER, BYLINE: The sky over the Pacific Ocean is looking pretty ominous - big dark gray clouds in the distance.

I think it feels like rain.

KIM PRATHER: It's coming in. I think it's the front. I think it's what they call the cold front that's coming.

SOMMER: Sure, everyone wants it to rain, but Kim Prather especially. She's an atmospheric chemist at the University of California, San Diego who studies what makes it rain. We're next to a large truck trailer full of scientific gear that's parked on the coast about an hour north of San Francisco. You'd think it would be measuring water vapor. That's what a cloud is, after all, millions of water droplets. But Prather is looking at tiny particles in the air.

PRATHER: Pollution, sea spray, dust, smoke.

SOMMER: They're the seeds of a rainstorm. The water inside a cloud sticks to them, growing larger and larger until you get a raindrop or snowflake. Large pumps on the trailer next to us are pulling in millions of these particles so they can be examined one by one.

PRATHER: We didn't expect there to be much dust here. But, in fact, there are parcels of dust continuously coming across the Pacific right now.

SOMMER: It starts as massive dust storms in the deserts of Asia and Africa that gain altitude over the ocean, taking about week to make it to California. Prather has seen this before when she flew through Sierra Nevada snowstorms in a small research plane.

PRATHER: So days with dust, we found, were days where you had the most snow on the ground consistently. And it's not a lot of dust. It's just the right amount of dust that comes in and seeds the very top of the clouds.

SOMMER: In one study, Prather found that the right kind of dust storm could boost snowfall in California's mountains by 40 percent. It happens because dust helps ice crystals form.

PRATHER: Because ice is a very picky process. Ice only likes to form on certain surfaces. Dust seems to be very good. Bacteria, biological particles appear to be very good.

SOMMER: Yep, bacteria. Microbes that hitchhike on dust particles in the atmosphere.

PRATHER: Dust will come out of the ground with microbes on it. And so there are microbes that are still alive in those clouds.

SOMMER: But Prather says some particles have the opposite effect. Air pollution coming from California and coming over from Asia seems to reduce precipitation in the Sierra. Prather says she's trying to tease out these effects because it could help weather forecasting.

PRATHER: You know, are we getting more precipitation? Are we getting less precipitation? The ultimate goal is to be able to feed this into weather forecast models and improve those models, where they actually take into account the seeds. Right now, they don't.

BILL LAPENTA: From an operational perspective, we are giving that very serious consideration.

SOMMER: Bill Lapenta runs the weather forecasting division of the National Weather Service. He says today's weather forecast models are giant simulations that run on supercomputers. Adding dust particles to those models adds cost.

LAPENTA: It adds an additional amount of complexity, which then requires more computational resources.

SOMMER: And they can't get it wrong. The public relies on the forecast to prepare for extreme weather and floods. Still, Lapenta says they're hoping to add the effects of dust storms to their models in the next five years. Given how important weather prediction is in times of drought, it could be a big help when the next one hits. For NPR News, I'm Lauren Sommer in San Francisco. Transcript provided by NPR, Copyright NPR.