When it rains it's spores

by John D. Cox | May 29, 2009

Clouds may be the most obvious manifestations of weather, but there are a lot of things about them that are not at all obvious. 

On the scale of climate, their overall effect is the subject of abiding uncertainty.  High clouds tend to reflect incoming solar radiation, cooling the planet.  Low clouds absorb outgoing long-wave radiation, warming the earth.

On the scale of weather, they seem even more mysterious.  Researchers recently flew into high nascent clouds over Wyoming to snatch individual ice crystals -- analyzing them on the fly -- and they have come away with some surprising results.

Fully a third of the minuscule material that the ice forms around is not the dust and other inorganic flotsam and jetsam of the planet that you might expect but rather is stuff of biological origin -- bacteria, fungal spores and plant material.  The biosphere is sending up aerosols that are making rain, a circumstance that raises all kinds of interesting feedback possibilities between the biosphere and climate.

Last year in the journal Science, Brent Christner of Louisiana State University and colleagues reported a large fraction of microorganisms evidently served as "nucleating particles" in freshly fallen snow they examined around the world and wondered what it meant for the propagation of various plant pathogens.

Reporting this month in Nature Geoscience, Kerri Pratt and colleagues at the UC San Diego and elsewhere took the idea a step further and mounted an airborne mass spectrometer in a research aircraft and took it into the clouds.  As the pie chart illustrates, 33 percent was of biological origin, and most of the organic and inorganic aerosols over Wyoming were traced to Asia and Africa -- indeed, to specific dust storms.

The mass spectrometry process doesn't make possible the identity of the biological particles beyond their organic nature, nor can the scientists tell whether any of the microorganisms were alive in the clouds over Wyoming.

- John D. Cox
IMAGES:  National Center for Atmospheric Research, Nature Geoscience