By Janet Raloff
Bees and their nectar-loving friends aren’t the only means of transporting pollen long distances. Water molecules grab some wind-strewn pollens and whisk them up into the clouds, where they can become the seeds on which ice crystals form. These tiny bits of plant tissue can raise the temperature at which cloud water will freeze by up to 18 degrees Celsius, a new study finds — increasing the reflectivity of clouds and likelihood that droplets become heavy enough to rain out.
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This ice crystal formation, called nucleation, is made possible not by the pollen itself but by water-soluble chemicals on some pollens’ surface, suggesting that biological material may not be required to seed clouds, scientists from Austria and Germany report online October 5 in Atmospheric Chemistry and Physics Discussions.
Researchers have long known pollen grains can reach some clouds. But these carriers of male reproductive cells — the plant equivalent to sperm — are relatively large and heavy, notes microbiologist Brent Christner of Louisiana State University in Baton Rouge. So few scientists had suspected that enough pollen could reach clouds to play a major role in cloud chemistry.