Year after year, humid, storm-prone areas of the southeastern United States rack up top scores for the number of lightning strikes that flash across their landscapes. Now researchers say industrial pollution is part of the reason. The findings reveal another way in which human activities can affect local weather patterns.
Cloud-to-ground lightning occurs more frequently in urban areas, particularly in the southeastern U.S. Previous studies by Richard Orville and Scott Steiger of Texas A&M University in College Station have pointed to three possible causes: the effects of sea breezes over coastal cities, heightened air temperatures due to urban heat build-up, and pollution.
To test whether pollution might play a key role, the researchers mapped 14 years of lightning strikes for Louisiana along with the locations of oil refineries and other industrial sites across the state. "The pollution sources matched up so well with the lightning concentrations, it was like a bull's-eye," Steiger says. As reported in the 3 October issue of Geophysical Research Letters, the plot of 72 million lightning flashes showed three distinct regions of very dense activity. Two of these were just downwind of industrial regions around Lake Charles and Baton Rouge, relatively small inland cities where urban heat and sea breezes are likely minimal contributors to lightning activity, the researchers say. Sea breezes may contribute to the high incidence of lightning in the third area closer to the coast, however.
So how can air pollution cause lightning? Cloud droplets form around aerosols--suspended particles of pollution--and get swept high up into clouds, where they freeze instead of coalescing and forming rain, Steiger says. The static charge the particles carry with them sets the stage for the electrical conditions that detonate lightning.
Earle Williams, an atmospheric scientist at the Massachusetts Institute of Technology, says the results clearly suggest that aerosols play a role in enhancing lightning. However, he says, "Louisiana's oil refineries operate around the clock, so you wouldn't expect the aerosol concentration--and hence its effect on lightning--to vary much over 24 hours." Yet, he points out, the study found the highest frequency of lightning in the daytime, when the urban heat effect is strongest. More work is needed to pin down pollution's relative role, Williams says.