What’s the News: Bacteria are everywhere—in us, on us, around us. But they’re also floating around in the atmosphere, and researchers cracking open hailstones have now discovered them at the core, lending credence to the theory that bacteria jump-start the atmospheric process of forming snow, hail, and rain as a way to hitch a ride down to Earth.

What’s the Context:

We’ve long known that bacteria are involved in precipitation—scientists have been studying the idea since the 60s. But only in recent years have we begun to piece the relationship together.
In 2008, a landmark study analyzed the bacterial populations of 19 snowfalls, running tests to see how the microbes affected the freezing of water. They found that some bacteria dramatically increased the temperature at which ice crystals form; for instance, in the presence of Pseudomonas syringae, ice crystals, which normal require a frigid –40 C, can grow at a balmy –2 C. Bacteria could be catalyzing the formation of ice in the atmosphere, helping cause precipitation. And such organisms are pervasive: the team found bacteria in snow from all over the world. The lead researcher of that study, Brent Christner of Louisiana State University, told ...