How Clean Air Act Made Atlanta Rains Rebound

"It suddenly just changed dramatically in the '70s. It wasn't a gradual change. It was pretty abrupt," said Jeremy Diem, a climatologist at Georgia State University who performed the research.

His findings, he added, showed that the pollution controls were the main driver of the increased rainfall, although other scientists have argued there were different factors. "Other people said we had a recession and that caused less fuel to be consumed," Diem said, adding that that was likely also a factor.

Reducing cloud efficiency

Particles like those emitted by power plants, as well as natural ones like dust, serve as what are called cloud condensation nuclei when they are suspended in air. Water molecules in the atmosphere glom on to the particles and can grow to create droplets of different sizes that can become clouds, rain and other forms of precipitation. Smaller particles have smaller water droplets surrounding them, while larger ones attract larger droplets.

Pollution can lead to suppressed rainfall because it makes a cloud less "efficient," Diem said, explaining that clouds formed from small particles are of an almost uniform particle and water vapor size. Clouds require particles of different sizes before they will rain.

"You don't want tons of little ones, which is what Atlanta had in the '50s and '60s," he said.

In the five years after the passage of the Clean Air Act in 1970, emissions of particles with diameters of 10 micrometers or less decreased by about 40 percent nationwide, according to the Environmental Protection Agency.

Decades of data

Diem was interested in localizing the effect of the reduced emissions to his home city of Atlanta. The city's location — not near a body of water or mountains — allowed him to obtain information from a ring of nine weather stations at similar elevation surrounding the city, as well as several within or near the core.

He hypothesized that rainfall outside of the city core would not be as affected by pollution because it was away from many sources of emissions, including those of cars or factories. On the other hand, pollution over Atlanta's downtown and suburbs would be more highly concentrated, leading to less rainfall.

Diem collected data from 18 National Weather Service co-operative stations during the summer months (June to August), with data ranging from 1948 to 2009. Diem then used the results at the rural locations to predict rainfall values at each of the urban and suburban locations.

"This produced expected rainfall values," he said. "The expected rainfall values were then compared with observed rainfall values."

Urban suppression

Diem's data showed rainfall in the urban core was suppressed before the Clear Air Act. The data, when compared with rainfall totals at the outlying stations, showed the core was reduced by about 40 millimeters (1.6 inches) in the 1950s and 1960s, when pollution was at its peak.

Atlanta's core then saw a resurgence to previous summer rainfall levels of 300 millimeters (11.8 inches) in the 1970s and more particularly, in the 1980s, that has remained consistent through to 2009, the latest year studied. [Weird Weather: One Strange Quiz]

The research was published in the August 2013 edition of the journal Atmospheric Environment.

For future studies, Diem plans to examine other large U.S. cities that also would have a ring of weather stations surrounding them. The most likely candidate cities would be located in the Midwest, he said. Possible locations could include St. Louis, Mo., Dallas/Fort Worth, Minneapolis/St. Paul, Pittsburgh, Cincinnati, Indianapolis, Memphis, Tenn. and Oklahoma City. Diem expects to see similar results to Atlanta in those cities, though only studies will tell for sure.

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