Here’s an in-depth look at the science of dust and precipitation from Douglas Fox writing for the High Country News. Click through and read the whole article. Here’s an excerpt:

Grass on the sand dunes dawdles in a breeze. The air drifting in from the Pacific Ocean is clear and cool on this gray February morning. But Kimberly Prather is not outside inhaling its salty tang. Instead, she stands in a windowless trailer parked behind the dunes, experiencing the air second-hand as it filters through a tangle of humming machines, tubes and wires.

“It’s just clean, clean sea spray,” she says, peering at a graph on a monitor. The line is mostly flat, punctuated by several sharp peaks. “That’s sodium chloride,” she says pointing at one. Then, on another graph, she notes a set of jumbled peaks for silicon oxide, aluminum and sodium chloride. “This is dust mixed with sea salt,” she says — “a hint that the dust is coming from pretty far away.”

The air may seem clear at Bodega Head, 40 miles up the coast from San Francisco — but thousands of particles smaller than red blood cells drift in each cubic foot. Some have traveled a single mile; others, thousands. Prather’s machine sniffs them one by one, with a precision that no human nose can match. Its laser vaporizes five or six per second, spattering their guts across an ion detector, producing the chemical barcodes she peruses. A digital tally twirls alongside, like an odometer, too quickly to read — over 4,600 particles vaporized so far this morning.

Prather, an atmospheric chemist with the Scripps Institution of Oceanography at the University of California, San Diego in La Jolla, displays a similar frenetic energy as she narrates the second-by-second stream of chemical barcodes popping up on her machine. “That’s a really good one, a lot of magnesium,” she says to me — then, into her phone, “Sorry, I’m showing him data.”

Cloistered in this compartment, glasses pushed back over curly brown hair, she is like a sibyl who can see far away, reading at a glance the stories told by each obliterated particle. She knows when a ship, invisible over the horizon, is burning dirty fuel by the trace of vanadium it leaves. Peaks for two isotopes of lithium, meanwhile, are “almost a ringer for Asia,” she says, indicating crushed coal incinerated by Chinese power plants. She knows these things because she has analyzed samples from hundreds of sources around the world.

Prather has toured the Western U.S. with her machines too, visiting coastlines, mountains, farms and even the suburbs of Southern California, where she chased semi trucks down Interstate 215. Her work on airborne dust and particulates could help answer a question critical to the drought-prone West’s future: What causes a cloud to drop rain or snow? And why does one cloud weep uncontrollably, while 20 others shed hardly a tear?

A cloud bulging above a mountain crest can hold several million pounds of water, but that water is divided into droplets too small to overcome the atmospheric updraft lifting them. Only rarely can liquid droplets grow large enough to fall on their own. Most of the time, they have to freeze into ice to achieve the kind of runaway growth — snowball, you might say — into something large enough to fall back to Earth as rain or snow. But freezing is harder than you might think. Never mind the 32 degree Fahrenheit freezing point that you were taught in school: Tiny water droplets can remain stubbornly liquid at temperatures as low as 20 or 30 degrees below zero, perched on a thermodynamic cliff.

Scientists have long believed that the instigator that tips them over the edge, into becoming ice, hides somewhere within the microscopic flotsam floating in the air. They have searched for it for decades. Now, Prather and her colleagues are finally closing in on its identity.

Ironically, this new information about the tiniest particles is changing our view of how the world works on a grand scale, revealing a vast network of airborne dust commerce that ties together far-flung parts of the globe. These connections have enriched the West’s soils and shaped its precipitation patterns for thousands of years. But human-driven global warming and pollution have already begun to alter them, with implications for everything from agriculture and drinking water supplies to the basic shape of Western ecosystems. “We don’t understand this complex, totally intertwined system,” says Prather. “But we need to understand it soon.” [ed. emphasis mine]

[…]

TThat Asian dust triggers Western rain is a momentous discovery. Still, a major question remains: What exactly is it about this dust that triggers ice formation?

The simplest explanation is that specific kinds of mineral crystals provide a good scaffold for water molecules to latch onto, as with Bernard Vonnegut’s silver iodide. Experiments published in 2013 show that one class, called K-feldspars, trigger ice especially well. Although K-feldspars comprise only a minor component of dust worldwide, they are consistently found in dust from the Taklamakan Desert.

There’s likely more to it, though. Hitchhikers on the dust’s surface may play a key role. Back in the trailer at Bodega Head, a set of conspicuous peaks for carbon-nitrogen and carbon-nitrogen-oxygen catches Prather’s eye. “A biological marker,” she says, “a piece broken off a protein” — most likely the incinerated innards of a living cell that had been riding on the speck of dust she just zapped.

Forty percent of the Asian dust she’s sampled over Wyoming and California contains such signs of life. Other scientists have found that desert microbes can survive transoceanic transport on grains of dust. The same traits that allow them to inhabit hot deserts also provide protection against the aridity and ultraviolet radiation of the upper atmosphere, an advantage that may allow them to ride the global dust train from one continent to new habitats on another.

A few microbes even secrete proteins that nucleate ice — some even more potently than silver iodide. And scientists collecting ice-nucleating particles from snow in Montana have found that heating them to bacteria-killing temperatures or treating them with a bacteria-obliterating enzyme — processes that don’t alter mineral structures — sharply reduces their ability to create ice.

But other, tinier vermin could be even more important — hitchhikers on the hitchhikers. Sometime after noon at Bodega Head, Prather leans close to inspect a barcode with a towering peak of phosphorous — a classic signature for something containing lots of DNA and not much else. “Oh, my goodness,” she says. “That was the viruses.”

Any dust that is carrying bacteria is just as likely to carry these ubiquitous genetic parasites. And while bacteria have squishy membranes that may provide poor footing for water molecules — like stacking Legos on a waterbed — many viruses are sealed in rigid protein cases. Those cases are covered in a repeating tile pattern of positive and negative charges that water molecules might just latch onto with ease. “Maybe,” says Prather, “that pattern is magical.”