One of the best science essays I read last year was this one by Times Blogger Olivia Judson on the possibility of clouds being an unrecognized and unexplored ecological niche. The Wild Side, New York Times An excerpt: "Clouds. It’s been known for ages that microbes — bacteria, algae, fungi, and other tiny organisms — can be found in clouds. This isn’t surprising. Microbes often get airborne. They can be lofted by the wind from the leaves of a plant; or thrown into the air when a bubble of water bursts, and then lofted by the wind. They get high. They’ve been captured in the mesosphere — that’s the layer of atmosphere above the stratosphere — as much as 70 kilometers (more than 40 miles) above the Earth’s surface. And microbes regularly travel long distances by wind. Indeed, in 1832, Charles Darwin, at sea on the “HMS Beagle,” noticed that dust had landed on the ship; and from the position of the wind and the ship, concluded that it must have come more than 500 kilometers (300 miles) from the coast of Africa. He collected it, and sent it off for analysis; it turned out to contain numerous species of African freshwater algae. Similarly, clouds have been suggested to be a way that microbes get around the planet — a sort of bus system for bacteria. But the paper in “Geophysical Research Letters” went further. It claimed not just that microbes are traveling via cloud, but that some of them are actually living there — growing, metabolizing, reproducing — until plummeting back to earth when the cloud rains."