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Is air pollution shifting rain away from weekends?

Pollution particles might be seeding the clouds and goosing rainfall.

So maybe not all of the effects of greenhouse gases are bad.

Air pollution, which peaks midweek because of traffic, tends to cause storms to shed more rain during the week than the weekend. Scientists at NASA's Goddard Space Center recorded summer rainfall data between 1998 and 2005 in the southeastern U.S. with a satellite and cross-checked it against data on airborne particulate matter.

The researchers found that it rains more, on average, between Tuesday and Thursday than from Saturday through Monday. Afternoon rainfall peaks on Tuesday, which gets 1.8 times as much rain as Saturday, when the least amount of rain falls.

Pollution may be seeding the clouds. Particles injected into clouds attract moisture and form raindrops. Others dispute this, stating that increased particulate matter spreads the moisture over more particles and thus hinders rain.

Either way, data from 2007 echoes the weekday trend.

"If two things happen at the same time, it doesn't mean one caused the other," said Thomas Bell, an atmospheric scientist at NASA's Goddard Space Flight Center in a prepared statement. "But it's well known that particulate matter has the potential to affect how clouds behave, and this kind of evidence makes the argument stronger for a link between pollution and heavier rainfall."