As if terrorism, pollution, and crime aren't enough to drive people from cities, urbanites also have to deal with a lot more lightning than do those living in the countryside, says Kleber Naccarato, an atmospheric scientist with Brazil's National Institute for Space Research in São José dos Campos. He used Brazil's extensive lightning-detection network to measure summer flashes over three large metropolitan areas. Naccarato and his colleagues found that cloud-to-ground lightning—the most common type—was nearly twice as common over large cities as over surrounding areas.
In urban, sparsely vegetated areas, such as much of São Paulo, heat emitted by buildings, cars, and pavement creates heat islands where temperatures are higher, clouds are deeper, and lightning is more likely, the researchers say. Air pollution also increases the frequency of lightning flashes by seeding the electrification of thunderclouds. And for reasons that are not clear, positively charged bolts struck less frequently when the levels of silicon dioxide and other aerosols in the air were high. "Lightning activity tends to concentrate where the human activities are more intense, which increases the potential harm," Naccarato says. His findings may help by clarifying the areas where lightning is most likely to occur.