To look at the false-colored U.S. map of cloud-to-ground lightning flashes over the past decade, you would think that someone had planted a huge lightning rod in the middle of Houston. During peak thunderstorm season (June to August), the city is hit by an average of 1,700 ground flashes a month¿only areas in Florida are hit worse. And there are twice as many ground strikes over and immediately downwind of Houston as there are upwind just 80 kilometers away.

"Somehow 4.5 million people are having a major effect on the meteorology of Houston," says Richard Orville of Texas A&M University, lead author of a paper to be published in Geophysical Research Letters. The researchers relied on the National Lightning Detection Network, a database that pinpoints ground flashes with unprecedented accuracy. A 1995 study of 16 Midwestern U.S. cities used these data and found a correlation between city size, air pollution and lightning, but it could not single out one factor responsible for the extra lightning, which was generally much less than in Houston.

This article was originally published with the title Bright Sky, Dirty City?.

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