Rainy Days Driven by Traffic Patterns, Study Says

Richard A. Lovett
for National Geographic News
July 13, 2006
Think it rains only on the weekend? Not if you live in the Southeast
United States.

Summer rainfall in this region of the country appears to mimic the highs and lows of air pollution from weekday commuters, says Thomas Bell of the NASA Goddard Space Flight Center in Greenbelt, Maryland.

At the May meeting of the American Geophysical Union in Baltimore, Maryland, Bell reported that afternoon thundershowers are more frequent and more intense on weekdays than on weekends.

Bell limited his study to summer thunderstorms, which, in theory, are most likely affected by changes in air pollution.

Meteorologists believe smog contains tiny particles that spur the formation of water droplets, which eventually become raindrops.

More smog, therefore, not only means more droplets, but also tinier ones—at least in the initial stages of storm formation.

These smaller droplets are carried higher into the air before falling as rain, which ultimately increases storm intensity.

(Related story: tornado chasers in National Geographic magazine.)

Wednesday Thunderstorms

Scientists have long speculated that pollution from weekday commuters might affect the storm cycle. But previous studies failed to detect a link.

Bell says that past studies tended to focus on individual cities, particularly those in coastal areas where other factors may also influence storms.

Seeking to examine the issue more broadly, Bell analyzed rainfall patterns from nine years of satellite data from the Southeast quadrant of the U.S.

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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