Do you remember seeing clouds from an airplane for the first time? Even if that first time was as an adult, you were probably struck by the appearance of solidity. Seen from above, a cloudscape looks like a landscape — it looks like a place where things might live.

At school we learn that clouds aren’t solid: They are just made of water vapor. And when the amount of water in a cloud reaches a certain point, it becomes too heavy to stay suspended in the air, and falls down: It rains. The process, we are told, is a physical one. Condensation, cooling, saturation, precipitation.

How thrilling, then, to learn that the world is a more complicated place. To a whole range of organisms, clouds are places to live. To microbes, clouds are not just landscapes: They are ecosystems. Even more than that, precipitation — the act of rain and snow falling out of the sky — seems to be biological. Rain, you could say, is bacteria’s way of getting out of the sky.

Kim Prather at the University of California, San Diego, studies these aerial ecosystems. She and her team fly in special research planes over the Pacific Ocean off the west coast of the United States. You may have seen biologists chasing butterflies and dragonflies with insect nets; what Prather is doing is the equivalent in a cloud. Her team takes samples of clouds and analyzes the content.

“We’re seeing lots of biological components such as bacteria and molecules associated with microbial life,” she says.

It’s not the first time microbes have been found to be present in the atmosphere. A few years ago I interviewed Brent Christner of Louisiana State University, Baton Rouge. He had collected fresh snow from diverse locations in the United States, France and Antarctica, and in all samples he found evidence that bacteria were not only present, but that they had been influential in actually causing the snow to fall in the first place.

How on Earth can that be?

Rain falls when something “seeds” the development of ice crystals in a cloud. Tiny particles cause ice to form and grow, and eventually it will fall. If it warms up enough as it falls, it will be rain. Sometimes ice crystals themselves are the catalyst for growth, and sometimes ice grows around tiny particles of dust.

Prather’s group is directly sampling from the clouds over the Pacific to find out more details. Last week she presented her work at the American Chemical Society meeting in San Francisco.

Her starting point is the understanding that the atmosphere is chock-full of dust and other particles, and that bacteria, algae and fungi live there too. Understanding the exact chemical makeup of the dust and the biological molecules on it helps understand and predict how rain and snow fall from clouds. How, when and how much — crucial information for farmers, and for us all.

“The standard belief is the more ice you have in a cloud, the more likely you will get precipitation out of it,” Prather says. “Our goal is to catch the first stages of ice forming and find out what exactly the chemical constituents are that the ice is forming on.”

What they have found is that the ice crystals have biological markers. They have proteins that can’t be derived from dust particles in the air, but that are signatures of bacterial life in the atmosphere.

“We’ve learned that not all of the particles in the air at high altitudes have the same influence on clouds. We’re starting to think that these differences contribute to how rain gets distributed,” says Prather.

Most of the dust that Prather’s team detects in clouds and precipitation originates in deserts in Asia. It gets swept westward by the jet stream, where it mixes with other airborne particles, including smoke and spray from the sea. Prather says that each of these types of particles — collectively known as aerosols — has its own, distinctive impact on clouds.

But living on the particles are varieties of microbes.

The microbes make proteins, which lace water molecules together. The water forms a pattern similar to an ice crystal’s lattice, which encourages ice to form. Ice crystals then grow in the normal way, and rain — or snow — falls.

A long-term goal of Prather’s research is to improve cloud-seeding technology. This was most publicly used just before the Beijing Olympics in 2008 to ensure clear skies for the opening ceremony. But the techniques are not always reliable. “Mother Nature has developed very effective ways to seed clouds, so perhaps we could take some tips from her,” says Prather.

Evidence, if any more was needed, of the extraordinary power of natural selection. All organisms need to be able to disperse and find new areas to live. After staying alive and reproducing, dispersal is the third most important item on any organism’s to-do list. It seems a range of organisms have found a way to manipulate weather systems to help them do that.

There are occasionally reports of masses of frogs raining out of the sky, or of fish falling in the desert. In Kerala, in southern India, there was an infamous occurrence of red rain, which had some people speculating that alien life forms had rained down from space when a meteor exploded in the atmosphere. It turned out to be red algae that had been swept into the air after a storm, just like when frogs and fish are swept up on freak air currents.

But the finding that there are organisms that live for at least part of their life cycle in the atmosphere — that is as wonderful as stories about magical creatures that live in the clouds, and all the more impressive for being true.

Rowan Hooper (@rowhoop on Twitter) is the news editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”

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