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Soot and dust were long considered the best candidates for these one-in-a-million particles. But the discovery that certain bacteria carry a gene that allows them to form ice, along with the realization that the skies are teeming with microbial life, has made some researchers look to airborne biology for answers. "The amount of different types of microbial life present in the cloud droplets that make up a winter storm is amazing," says Gary Franc, a microbiologist and plant pathologist at the University of Wyoming in Laramie. "There's a whole ecosystem going on in the clouds that's largely undefined."
Scientists like Franc are only now starting to catalog what could be thousands of species of microbes drifting in the sky, many of them almost certainly new to science, and some perhaps capable of surviving high in the stratosphere, where conditions are roughly as favorable for life as they are on Mars. Prather's studies suggest that dust from Asia and Africa might carry ice-forming microbes all around the globe.
All told, up to two million tons of bacteria may find their way into the atmosphere each year, not to mention 55 million tons of fungal spores and unknown quantities of algae. They have been overlooked for decades, but scientists are finally recognizing how diverse this atmospheric ecosystem is and — perhaps — how much influence it could exert over tomorrow's weather or next year's harvest.
The Inner lives of clouds
Clouds feel utterly familiar to us. We idealize them as tufts of cotton drifting overhead, curse them for ruining a picnic, or just take them for granted. But clouds possess inner lives, marked by complex, second-by-second ebbs and flows.
A cloud over a mountain may look stationary, but it more closely resembles a standing wave in a river — an atmospheric river. As moist air rises over a range like the Sierras, falling temperatures cause water vapor to condense into droplets, giving rise to clouds. New droplets form in the cloud every second. But any given droplet lives for only an hour before the current sweeps it down the far side of the mountains, causing it to re-warm, evaporate, and vanish. Only an ice crystal can grow quickly enough to hit critical mass and fall during that short time. Ice doesn't form easily, though.