Dirty Weather

May 27, 2011

Rheanna Sand


This time of year, we prairie-dwellers of North America expect severe weather. Hot days, high winds, and the mixing of warm and cool air create a rich breeding ground for thunderstorms, hailstorms, and of course twisters, which have been touching down in droves over the past week in the part of the U.S. affectionately known as "tornado alley." Most people see the weather as being controlled by large scale phenomena like the water cycle, the coriolis effect, and the changing of the seasons. However, new evidence suggests that the precipitation of water vapour into rain droplets, snowflakes, and hailstones, and even the formation of clouds themselves, could be due to bacteria.

This idea of "bioprecipitation" is being looked at by a group of researchers at Montana State University, who collected hailstones five inches in diameter or larger and analyzed the bacterial content of the different layers. They found that the bacterial count was highest in the center, supporting the idea that the bacteria are acting as an initial seed on which ice crystals can grow. This seeding effect explains why hailstones can form at temperatures too warm for spontaneous ice crystal formation (-40 degrees Celsius and higher).

In theory, any small particle could act as an ice nucleus, but bacteria seem to be the most effective. In one species, Pseudomonas syringae, this can be attributed to the fact that their outer coat attracts water molecules and binds them in a regular pattern, priming the crystal formation process.

High numbers of bacteria in the atmosphere can affect cloud cover, the amount of rain, and even the insulation of the Earth from solar radiation. Given that bacteria can be found in water, in soil, and even in your gut (about four pounds worth!), it begs the question: is there anywhere bacteria won't go?



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