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NASA has determined that air pollution provokes wetter weekdays and drier weekends in the southeastern US, with peak rainfall occuring late on Thursdays provoked by increased levels of airborne particles.

That's the conclusion of a study of summertime storms using data collected from the agency's Tropical Rainfall Measuring Mission satellite, aka TRMM.

Scientists at NASA's Goddard Space Flight Center, led by atmospheric scientist Thomas Bell, used the data to "estimate daily summertime rainfall averages from 1998 to 2005 across the entire Southeast", and discovered that it "rained more between Tuesday and Thursday than from Saturday through Monday".

The summer 2007 satellite data backed the initial findings, demonstrating that peak rainfall occured "late on Thursdays". Midweek increases in rainfall were generally, however, "more significant in the afternoon, when the conditions for summertime storms are in place" - peaking on Tuesdays which enjoyed 1.8 times more rainfall than Saturdays.

The team then obtained US-wide particulate matter figures for 1998 to 2005 from the Environmental Protection Agency, which showed that "pollution tended to peak midweek, mirroring the trend observed in the rainfall data".

Bell says: "If two things happen at the same time, it doesn't mean one caused the other. 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."

The effect of pollution on the summertime storms is believed to be that the particulate matter "seeds" storm clouds, although some maintain this pollution-based seeding "thwarts rainfall by dispersing the same amount of water over more seeds, preventing them from growing large enough to fall as rain".

However, NASA explains: "In the Southeast, summertime conditions for large, frequent storms are already in place, a factor that overrides the rain-thwarting dispersion effect. When conditions are poised to form big storms, updrafts carry the smaller, pollution-seeded raindrops high into the atmosphere where they condense and freeze."

Bell elaborated: "It's the freezing process that gives the storm an extra kick, causing it to grow larger and climb higher into the atmosphere."

Bell concludes by describing increased midweek rainfall as "a tendency" rather than an absolute, but reckons the team's findings might be used to improve rainfall forecasts which "probably under-predict rain during the week and over-predict rain on weekends".

The researchers' study is published in the American Geophysical Union's Journal of Geophysical Research. ®

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