COMMUTERS' car exhaust doesn't just warm the globe - it can also increase lightning strikes for miles around.
During the working week, air pollution rises because of all the vehicles on the road. This effect has been shown to modify rainfall patterns both at the weekend and during the week by creating stronger updrafts of air and bigger clouds.
Now it seems weekday pollution can bring lightning as well as rain. Daniel Rosenfeld of the Hebrew University of Jerusalem, and colleagues, counted strikes recorded across the US by the ground-based National Lightning Detection Network in June to August, from 1998 to 2008.
In the south-eastern states, lightning strikes increased with pollution by as much as 25 per cent during the working week. The moist, muggy air in this region creates low-lying clouds with plenty of space to rise and generate the charge needed for an afternoon thunderstorm.
Surprisingly, the effect was not strongest within big cities with high pollution, but in the suburbs and rural areas surrounding them. "There is a misconception that if you get away from cities, you get away from the pollution. Actually, it follows you for hundreds of miles," says Rosenfeld, who presented the research at the American Geophysical Union meeting in December.
He says the heat generated by urban areas may locally override pollution's effect on lightning.