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Bad Air
Fri Jul 08, 2005
Listen in RealAudio 
Smog, blackened buildings, respiratory problems, damage to sensitive ecosystems - the effects of polluted air are obvious. But did you know that dirty air can also impact rainfall? Hi, I'm Bryan Yeaton, and this is The Weather Notebook.
This was one of the surprising findings in a recent study of Mediterranean air quality, led by the Max Planck Institute in Germany.
The fact that the celebrated azure skies of the Mediterranean have some of the worst air pollution in the world is startling enough. Jos Lelieveld who headed the study explains that global circulation patterns make the Mediterranean a kind of pollution crossroads, with foul air converging from Europe, Asia, and even North America. Lelieveld's team also discovered that the worse the air quality, the less rain the Mediterranean receives in summer.
To understand why this happens, we need to think about the cycling of water between ocean and atmosphere. As the sun beats down on the dark sea surface, water evaporates and sea surface temperatures rise - both of which are conducive to rainfall. But when there are particles of pollution suspended in the air, less of the sun's energy reaches the surface of the ocean - resulting in less evaporation, lower water temperatures and less rainfall. The severe drought in the eastern Sahel around 1980 coincided with a period when Mediterranean pollution levels were at their highest.
The good, or at least less bad news is that Mediterranean air pollution levels have been dropping lately - and this combined with global warming has boosted both water temperatures and summertime rainfall.
Thanks to David Laskin for today's show. Our program is produced in New Hampshire at the Mount Washington Observatory, on the web at www.mountwashington.org. We are supported through grants from The National Science Foundation, and Subaru of America.
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