Photo from NASA's Earth Observatory of a dust storm in the Sahara Desert in northwest Africa.
Photo from NASA’s Earth Observatory of a dust storm in the Sahara Desert in northwest Africa.

It’s March 1, which means Northern California is past its driest January-February period on record. Of course, the long-range forecast looks dry, too. So our question, as state water officials report on a Sierra snowpack that has fallen far short of normal for this time of year: “Where’s our rain?”

OK–we don’t know the answer to that one. But there’s some surprising research out on where our rain and snowfall come from.

A study from a team lead by scientists from UC San Diego and the National Oceanic and Atmospheric Administration and published yesterday in the journal Science finds that dust and “biological aerosols” from Asian and African deserts play an important role in bringing snow and rain to California. (What’s a biological aerosol? A writeup on the study from the site Science Codex says “aerosols can be composed of sea salt, bits of soot and other pollution, or biological material. Bacteria, viruses, pollen, and plants, of both terrestrial and marine origin, also add to the mix of aerosols making the transcontinental voyage.”)

In short, the tiny bits of matter swept up by the winds swirling across the immense stretches of desert on the other side of the world ride high-altitude winds and eventually form the nuclei of ice crystals that in turn trigger snow and rain over California and elsewhere in the western United States.

Bettina Boxall of the Los Angeles Times says the research suggests a climate mechanism that could have a major impact on California’s water supply as Earth’s climate undergoes warming:

“I think it has huge implications,” said Guido Franco of the California Energy Commission, which funded the program. “It may counteract some of the effects of a warming climate.”

Scientists predict that, in general, Earth’s wet regions will become wetter with global warming and dry regions will become drier. That could mean more windblown desert dust in the atmosphere and, if the Sierra results bear out, more precipitation in the Northern California mountains that provide the state with roughly a third of its water supply.

Marshall Shepherd, a University of Georgia research meteorologist and president of the American Meteorological Society, was not prepared to go that far. “I don’t know if we can make that leap yet and say that [more dust] is going to lead to large global changes in precipitation patterns,” Shepherd said. “There are so many other competing factors.”

The research was part of a three-year project called CalWater, a joint effort by UC-San Diego’s Scripps Institution of Oceanography, NOAA, and the California Energy Commission.

California Water: From African Skies to Sierra Nevada Snow 29 March,2013Dan Brekke

  • Global warming means there’s less snow overall. But it could also bring more extreme snowstorms.

  • Marshall Shepherd was right not to agree (or disagree) with the idea that more nucleii forming dust would lead to more precipitation. This works only up to a point, then after too many nucleii are introduced they compete for the available water vapor and create many smaller drops–drops which may not reach the ground in some cases. This is one of the principles at work in cloud seeding.


Dan Brekke

Dan Brekke is a blogger, reporter and editor for KQED News, responsible for online breaking news coverage of topics ranging from California water issues to the Bay Area’s transportation challenges. In a newsroom career that began in Chicago in 1972, Dan has worked as a city and foreign/national editor for The San Francisco Examiner, editor at Wired News, deputy editor at Wired magazine, managing editor at TechTV as well as for several Web startups.

Since joining KQED in 2007, Dan has reported, edited and produced both radio and online features and breaking news pieces. He has shared in two Society of Professional Journalists Norcal Excellence in Journalism awards — for his 2012 reporting on a KQED Science series on water and power in California, and in 2014, for KQED’s comprehensive reporting on the south Napa earthquake.

In addition to his 44 years of on-the-job education, Dan is a lifelong student of history and is still pursuing an undergraduate degree.

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