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It's raining cats and bacteria

Ice crystals plucked from clouds and analyzed in flight show bits of biological material - bacteria, spores and plants - play a role in the formation of clouds, American researchers said yesterday.

The finding, reported in the journal Nature Geoscience, offers the first direct evidence of airborne bacteria in clouds, they said.

Climate scientists typically rely on computer models to predict climate change, but until now it has been difficult to directly measure the composition of ice crystals in clouds, which are the very seeds that form clouds.

"By sampling clouds in real time from an aircraft, these investigators were able to get information about ice particles in clouds at an unprecedented level of detail," said Anne-Marie Schmoltner of the National Science Foundation's Division of Atmospheric sciences.

"By determining the chemical composition of the very cores of individual ice particles, they discovered that both mineral dust and, surprisingly, biological particles play a major role in the formation of clouds."

"The key to cloud formation is these little seeds that feed the clouds," said Kim Prather of Scripps Institution of Oceanography. "We are basically trying to understand what is forming clouds."

The team, led by Prather's graduate student Kerri Pratt, found that biological matter accounted for 33 percent of the particles in ice crystals, and mineral dust accounted for 50 percent.


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