Pollution Causes Most Lightning Strikes at Midweek

Nov. 26, 2008 -- As if getting through "hump day" could get any tougher, new research shows that the middle of the week is the worst time for lightning.

There are 10-20 percent more strikes on Wednesdays and Thursdays than there are on the weekends. The weekly spike in dangerous weather appears to occur in the summer in the southeastern part of the United States as a result of air pollution.

"The one thing that seems to change most during the week is heavy truck traffic," Thomas Bell of NASA's Goddard Space Fight Center in Maryland said. "It starts rising on Monday, then peaks around Wednesday or Thursday. We don't know why that would be, but it's a pretty dramatic pattern."

In previous research, Bell found that summer precipitation in the southeastern United States follows a similar pattern. He suspects that microscopic particles spewed from trucks' diesel engines act as seeds for forming water droplets in clouds. With more seeds in the air more droplets form, but they are smaller.

Rising bubbles of warm air can easily loft small droplets high into the atmosphere, forming huge, towering thunderheads. And if the droplets get high enough, they freeze.

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"The thinking is that ice crystals colliding with one another plays a big role in the charge separation that causes lightning," Bell said, though he added that the exact mechanism is still a bit of a mystery.

Using data from the National Lightning Detection Network, Bell compiled lightning strike information from 1998 to 2006 over the continental United States. He found that in several regions -- and especially the southeastern United States -- summer lightning strikes went up significantly during the middle of the week.

The study is due to be presented next month at the American Geophysical Union meeting in San Francisco.

"People have been debating about the effects of aerosols on weather for the last five years or so," Dong Wu of the California Institute of Technology said. "We know there is a relationship between aerosol loading in the atmosphere and rainfall; it's one way humans seem to be influencing the climate."

A series of studies over the past few years have shown that surface temperature, cloud formation, and precipitation all appear to follow a weekly cycle that could be linked to human activity.

"Earth's climate is going to warm significantly in the next 50 to 100 years," Wu said. "We want to know what role aerosols play in this; is it a warming effect, cooling, or neutral? It's a hard problem, untangling anthropogenic influence on weather from what is just natural variability. Looking at the effects of aerosol pollution is one way we can do that."

Related Links:

National Lightning Detection Network

Discovery Channel: Storm Chasers

Goddard Space Fight Center