Rainy workdays

Natural History, May, 2008 by Stephan Reebs

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This summer, storm chasers looking for the heaviest average rainstorms in the southeastern United States should venture out midweek in the afternoons.

So suggests a study by NASA meteorologist Thomas L. Bell and five colleagues, who analyzed data gathered between 1998 and 2005 by the Tropical Rainfall Measuring Mission satellite, a NASA spacecraft that uses weather radar and microwave imaging to estimate U.S. rainfall south of latitude 40 degrees north. The team reports that in the southeast--where rainstorms cover more ground and disgorge more water than anywhere else in the satellite's purview--rainfall follows a familiar daily cycle: downpours in the afternoon. Intriguingly, the rain also follows a weekly cycle. Maximum rainfall occurs in the afternoons Tuesday through Thursday--when it's almost double that of Saturday, the day of least rain.

Now, guess what? Air pollution--aerosol particles released by traffic and industrial activities--also peaks midweek, according to monitoring programs by the Environmental Protection Agency. Correlation does not prove causation, Bell duly notes, but he points to the fact that aerosols keep cloud droplets small, allowing updrafts to lift them to greater heights. Higher in the atmosphere they freeze, releasing more heat, and thus intensifying storms. (Indeed, storm clouds grow higher in the middle of the week.) This phenomenon is particularly likely where air is moist and warm--exactly the conditions that prevail on summer afternoons over the southeastern U.S. (Journal of Geophysical Research)