Next time you're in a snowstorm, look up and get a face-full of bacteria. It turns out there are bacteria up in the clouds, and some of them actually create ice crystals.

NPR's Christopher Joyce explains.

CHRISTOPHER JOYCE: This all started in Montana. Brent Christner had a colleague there who was puzzled by some nasty plant bacteria that kept infecting his wheat crops. No matter what he did, he couldn't get rid of them. He suspected they might be airborne. So, Christner says, his friend cooked up a strange experiment involving a petri dish and an airplane.

JOYCE: He took an airplane and flew up to the sky and then sort of opened the window and held a petri plate outside and actually detected these bacteria in the atmosphere.

JOYCE: The bacteria, which normally live on plants, were falling out of the sky. That was over 20 years ago. Christner went on to become a microbiologist himself and also an expert in ice and snow. He knew that at certain temperatures, particles floating in the clouds cause water vapor to crystallize around them. He remembered the flying bacteria.

JOYCE: They really intrigued me by the idea that if these bacteria were blown into the atmosphere and actually got into a cloud, that they could induce precipitation.

JOYCE: So Christner packed up his skis and went looking. He collected snow and ice and melted it down. Sure enough, he found his particles or nucleators, as they're known among ice experts.

JOYCE: Most active ice nucleators in these samples are biological in origin. Every sample that we've looked at, we found them.

JOYCE: He found these ice-bugs all over the place - France, Montana, the Yukon, and in some pretty unlikely places, too.

JOYCE: We analyzed fresh snowfall from places like Antarctica, where there aren't any plants around. They were still present.

JOYCE: Christner says these bacteria have a special protein that gives them their ice-making powers.

JOYCE: It's a protein that mimics the lattice of an ice crystal, so it enhances ice crystal formation.

JOYCE: In fact, there are several kinds of bacteria floating around in the sky that could be making snow or even rain, according to Steven Lindow, a plant scientist at the University of California at Berkeley. He says scientists have suspected these bacteria may be using the atmosphere like an aerial freeway.

JOYCE: They could leave plants, reach the upper atmosphere, become ingrained in atmospheric particles that would fall later as rain or snow. This would bring them back to Earth, perhaps even as raindrops onto a new plant, where they could find a new home and start the cycle again.

JOYCE: He says the research, which appears in the latest issue of the journal Science, helps confirm that idea. Brent Christner, who works at Louisiana State University, adds that the findings pose an interesting possibility. For example, people might consider growing plants that harbor these bacteria in drought-prone places - in essence, he says, using a green thumb to make it rain.

Christopher Joyce, NPR News.

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