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More Rain From Less Pollution

11:00 PM, Jul 30, 2013   |    comments
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ATLANTA -- Feel like you are living in Seattle or the Amazon Rainforest lately thanks to the non-stop rain? You can thank the Clean Air Act for that -- well, sort of.

Back in 1970 the Clean Air Act was passed. This initiative focused on reducing airborne pollutants that posed a threat to human health.

Jeremy Diem, a climatologist at Georgia State University performed research looking at 18 National Weather Service co-op weather stations and noticed that Atlanta's average annual rainfall increased by 10 percent in the decade following the new Clean Air Act. He studied specifically the summer months during the years of 1948 to 2009.

"It suddenly just changed dramatically in the '70s. It wasn't a gradual change. It was pretty abrupt," explains Jeremy Diem, a climatologist at Georgia State University who performed the research. But not everyone agrees. "Other people said we had a recession and that caused less fuel to be consumed," Diem said, although he does agree that that was also probably a factor.

Diem explains that having a lot of pollution in the air can lead to clouds being "less efficient".

In general, any water molecule can create a cloud, but you need specific types of water molecules to make a cloud drop rain. Standard clouds form from small particles that are almost perfectly uniform in both particle and water vapor size. This is especially the case when a lot of pollution is factored into the clouds. However, in order for the cloud to create falling rain, the particles must be of various sizes.

"You don't want tons of little ones, which is what Atlanta had in the '50s and '60s," Diem said.
How does he know that? Well, the first 5 years after the Clean Air Act was passed, the recorded emissions of particles with diameters of 10 micrometers or less decreased by about 40 percent nationwide, that according to the Environmental Protection Agency.

Diem took his research one step further. He used the results he got from looking at the weather stations in rural areas to predict what the rainfall values would be at some of the urban and suburban locations. What the results showed was that rainfall in the urban areas was greatly suppressed before the Clean Air Act.

How much so?

The urban/suburban areas had rainfall totals of about 1.6 inches less in the 1950 and 60's (when air pollution peaked). In the 1970's those summer rainfall levels went back to the normal 11.8 inches. Starting in the 1980's the numbers started to become more consistent, and remained that way all the way through 2009 (the last year that was studied).

Diem's research will be published in the August 2013 edition of the Journal of Atmospheric Science, although he says he isn't finished. He would like to continue the study in other cities such as Minneapolis, Cincinnati, Dallas, Oklahoma City, and a few others.

For more on the EPA's Clean Air Act visit them online.

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