Bacteria can be at the center of what ice forms around, and then plummets back to Earth as rain, snow or sleet.

Brent Christner of Louisiana State University, with colleagues in Montana and France, reported in the journal, Science, that most ice nucleators, particles ice forms around, found in snow at mid- and high-latitude locations were biological in origin. I.e., it’s just just the yellow snow you need to worry about. It’s pretty much all filled with creepy crawlies (or, more appropriately, fearsome flagellum).

Their guess, then, is that the bacteria affects the rain cycle or actually causes their own precipitation.

The theory is that the microbes are swept off of plants (most found in the study were actually plant pathogens) by winds or just the drafts from farm tractors and propelled up into the atmosphere. Ice crystals form around the bacteria. More and more water freezes on the ice, and then down it goes. It lands on a plant (possibly kills it) and the whole cycle begins again.

“We think if (the bacteria) couldn’t cause ice to form, they couldn’t get back down to the ground,” said David Sands, Montana State University professor of plant sciences and plant pathology. “As long as it rains, the bacteria grow.”

Dust and other particles do fine for growing ice and causing rain below about 14 degrees Farenheit, but biological nucleators can cause condensation at warmer temperatures.

It’s not clear exactly what microbes are falling down on us, but Pseudomonas syringae, which infects wheat, corn and other crops causing damage when temperatures drop below freezing, is a contender. What that means for the average farmer is a potential new source of danger to crops–from above.

“As is often the case with bacterial pathogens, other phases of their life cycle are frequently ignored because of the focused interest in their role in plant or animal health,” said Christner. “Transport through the atmosphere is a very efficient dissemination strategy, so the ability of a pathogen to affect its precipitation from the atmosphere would be advantageous in finding new hosts.”