The Weather Notebook: City-Produced Clouds Can Reduce Smog

City-Produced Clouds Can Reduce Smog
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Dave Thurlow, Host

Not only is Mother Nature making clouds, but cities are making clouds too. And these clouds aren't just dumping rain or snow...they're helping to fight air pollution. Hi, I'm Dave Thurlow and this is The Weather Notebook.

An ongoing government study focuses on Atlanta and five other cities. It links the growth of urban areas to the growth of clouds. And what's surprising to the researchers is that these 'city-produced' clouds reduce smog, by as much as 33 percent.

The study found that the average daytime temperature in downtown Atlanta is 91 degrees, while the average daytime temperature in the Atlanta suburbs is 78 degrees. This creates what is known as an urban heat island, which acts the same way as an island in a stream. Air masses often split and flow around the heat island, and when the flows recombine, cloud development is enhanced, as well is the occurrence of thunderstorms.

The urban heat island effect is nothing new. But what is new is the discovery that the increase in clouds causes a decrease in smog.

A key ingredient in smog is what are called volatile organic compounds or VOC's. Car engines create "man made" VOC's, but the photosynthesis of trees and plants create "natural" VOC's, which account for up to half of the VOC's in a city.

Now, the clouds from urban sprawl reduce the natural production of VOC's by up to 68 percent because the clouds decrease plant respiration. Also, the rain that falls from the clouds washes away the smog. But the man-made VOC's are increasing.

So, the clouds aren't a cure all. Growing cities will create more and more smog. So, instead of relying on the clouds, the researchers are suggesting it just may be best to try to cool down cities to begin with. How? Well, to begin with, plant more trees and change roofs to light, sun-reflecting colors.

Thanks to Subaru and the National Science Foundation.