Rush-hour pollution in U.S. cities may be making work weeks soggier than the weekend, according to a new study with an excellently worded title: Midweek increase in U.S. summer rain and storm heights suggests air pollution invigorates rainstorms. NASA scientists studied rainfall from space (via a satellite called TRMM) and found that Tuesdays through Thursdays were noticeably wetter than Fridays through Mondays. Tuesdays were particularly damp, getting 1.8 times as much summer rainfall as Saturdays.
Though the relationship could be a coincidence, scientists do believe that cities can cause rain as hot concrete warms the air, tall buildings alter the winds, and pollution from car exhausts give airborne water something to condense on, forming raindrops. The effect seems to be most pronounced in the southeastern U.S., where there's lots of warm, humid air. See NASA's Earth Observatory for a fascinating exploration into the phenomenon of urban rain.
The new study suggests that hordes of commuters pump exhaust into the air during the week. Less-concentrated driving on the weekends leads to fairer skies - or at least, fewer mud puddles. (Bringing up exciting prospects for mass-transit campaigns: Buses for Blue Skies? Subway Riders Make Saturday Dryer?)
But wait a minute. Back in 1999, Arizona State University came out with a report that cities made the weekends wetter, not the weekdays. Their argument: the smog builds up during the car-choked work week, setting up big rainstorms for Saturday (not Tuesday). Who's right here? Or maybe things are just different in the arid Southwest.
(Bonus links: animated, up-to-the-hour global rainfall from TRMM - this will make you feel better about your local weather, believe me. And a lecture about raindrop shape from Drippy, a drop who works for the USGS.)