The Mystery of Rain Making Bacteria

It is no mystery that bacteria exist in the air. Airborne bacteria (and visuses) can cause a host of human illnesses including respiratory infections. But new evidence also suggests bacteria may play a large role in the precipitation cycle. Montana State University professor David Sands has studied the connection between airborne bacteria and precipitation. Along with his colleagues Christine Foreman, an MSU professor of land resources and environmental sciences, Brent Christner from Louisiana State University and Cindy Morris, the results have been published in Science (Ubiquity of Biological Ice Nucleators in Snowfall, free registration required to read the full article).

Sands and his colleagues examined precipitation from locations as close as Montana and as far away as Russia to show that the most active ice nuclei are actually biological in origin. Nuclei are the seeds around which ice is formed. Snow and most rain begins with the formation of ice in clouds. Dust and soot can also serve as ice nuclei. But biological ice nuclei are different from dust and soot nuclei because only these biological nuclei can cause freezing at warmer temperatures.

Biological precipitation, or the "bio-precipitation" cycle, as Sands calls it, basically is this: bacteria form little groups on the surface of plants. Wind then sweeps the bacteria into the atmosphere, and ice crystals form around them. Water clumps on to the crystals, making them bigger and bigger. The ice crystals turn into rain and fall to the ground. When precipitation occurs, then, the bacteria have the opportunity to make it back down to the ground. If even one bacterium lands on a plant, it can multiply and form groups, thus causing the cycle to repeat itself. Read more on cloud formation.

References: Montana State University and EurekAlert!, a service of AAAS