The region extends as far north as central Illinois and as far west as mid-Texas (map of the United States).
The scientist found that during June, July, and August afternoon thunderstorms were most common on Wednesdays and least common on weekends.
The showers exactly mirrored pollution intensity from vehicle traffic.
According to the satellite data, midweek afternoon rainfall was nearly double weekend precipitation. Weekday storms were also more likely to be intense downpours.
(Learn more about weather in National Geographic magazine.)
Driving Patterns
Bell and his colleagues say that atmospheric wind-speed data also indicate that stronger and more frequent storms occur on weekdays.
Local weather station rainfall measurements backed the team's findings.
Bell believes weekday commuter car traffic is unlikely to be the sole cause of the summer weather pattern.
While people change their driving pattern on weekends, they still drive, he says.
Monitoring by the U.S. Environmental Protection Agency has found that air pollution levels follow weekly cycles marked by mid-week peaks.
But there appears to be only a 10 to 15 percent dip in pollution on weekends.
Bell doesn't believe commuter car traffic alone is enough to explain the rain effect found by his study.
"Truck traffic drops off a lot on weekends," the researcher said. "So it might be something related to pollution from truck traffic. But that's a pure guess."
J. Marshall Shepherd is a geography professor at the University of Georgia in Athens who has also studied the effects of air pollution on rainfall patterns.
Shepherd applauds Bell's research in an email to National Geographic News.
"It's a very nice study," Shepherd said.
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