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 60s (when air pollution peaked). In the
1970s those summer rainfall levels went back to the normal 11.8 inches.
Starting in the 1980s 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 click here.
Sources: Live Science, National Geographic, Southern
Environmental Law Center, EPA
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