With winter still in effect, the enthusiasm with which one greets fresh snowfall has tended to diminish. Every snowflake may be unique and beautiful, but when millions of them are standing between you and your car, beauty can be ignored. Now a team of scientists has taken a closer look at the particulates that cause snow to form, and it seems that most of them are... alive.

Ice formation in the troposphere doesn't tend to happen on its own if the temperatures are higher than -40?C (coincidentally -40?F), but biological particulates can catalyze ice formation at higher temperatures by acting as a nucleus. A new study, published in Science, examines samples of fresh snowfall from sites in Montana, France, the Yukon, and Antarctica in order to get a better idea as to the makeup of those ice nucleators.

Compared to remote sites, snow samples from Montana and France contained higher concentrations of particulates--more than 100 per liter. Antarctica, by contrast, averaged just four parts per liter. Biological cells made up around one percent of these particulates, with Pseudomonas syringae being the most common bacteria.





DNA-stained cells of P. syringae (green dots) frozen within individual ice crystals. Image courtesy of Shawn Doyle and Brent Christner, Louisiana State University

Exposing the particulates to lysozymes—enzymes that disrupt cell walls—or 95?C heat reduced their ability to act as ice nucleators. Both of these treatments would affect living cells but not inorganic particulates such as dust, sand or soot.

One of the authors of the study, Prof. David Sands, postulates that forming rain or snow is part of the bacterial life cycle; "We think if (the bacteria) couldn't cause ice to form, they couldn't get back down to the ground," Sands said. "As long as it rains, the bacteria grow."

Sands suggests that changing bacterial populations may affect rainfall; for example, overgrazing during a summer could reduce the bacterial population, resulting in lower raindrop formation, although more work would need to be done to firm up this theory. Regardless, it is an interesting glimpse into the world of the snowflake.

Science, 2008. DOI: 10.1126/science.1149757