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Bad Air
Wed Jul 09, 2003
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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