Forget the gods - we shape lightning

Mythology considered lightning the plaything of "the gods." Science explained its atmospheric physics. Now research shows it also can be shaped by human hands.

Especially in cities.

Recent research suggests that crowded urban centers become "heat islands," whose relative warmth encourages thunderstorms. And pollution from cars and industries encourages lightning strokes.

It's "a very real effect," says Prof. Richard Orville at Texas A&M University in College Station, who has spent several years researching storms around Houston and looking at studies elsewhere. He says there's a higher density of cloud-to-ground lightning strokes in the Houston area than expected naturally and "it's caused by man."

Until recently, however, it wasn't clear which factor played the more important role in creating lightning - heat-island warmth or pollution. Thanks to a discovery made during an exercise in his physical meteorology class, Dr. Orville now thinks he has the answer: pollution.

The class researched 14 years of lightning data, looking for areas of enhanced activity. They found two - Lake Charles and Baton Rouge in Louisiana.

Lake Charles stood out. It's much smaller than Houston, so there's not enough human activity to produce a heat island or to generate much pollution from cars. Yet Lake Charles has as high a density of cloud-to-ground lightning as does Houston.

Orville and his student Scott Steiger soon found the reason.

Lake Charles's air is just as polluted with microscopic particles as is the air over Houston. The source is an industrial area with oil refineries west of the small city.

There's another such industrialized area in a band near Baton Rouge. As Mr. Steiger and Orville explain in their research in Geophysical Research Letters, their finding "strongly suggests that pollution plays a key role in lightning enhancement." They add that other urban influences "can be neglected."

Now Orville wants to know how the pollution effect works.

Meteorologists have a general idea of how thunderstorms generate lightning, but the details of what's going on in the clouds remain obscure.

Thunder clouds concentrate positive electrical charge in their upper region and negative charge lower down. Lightning flashes when the voltage buildup between cloud and ground is strong enough to break down the electrical resistance of the air.

Somehow, the polluting particles enhance a cloud's ability to accumulate electrical charges and build up that critical cloud-to-ground voltage.

Lightning scientists think that the polluting particles may act as nuclei on which water condenses to form cloud droplets.

This happens naturally in clouds, where droplets eventually coalesce into large drops that fall out. Electrical charges are generated along the way.

By flooding a cloud with condensation nuclei, pollution would produce more small droplets than is normal.

Many of these would be caught in updrafts and carried to heights where they would cool to temperatures below freezing and mix with ice crystals. Orville says scientists know that such a mix of supercooled water and ice produces electrical charging.

Orville will look more deeply into this possibility in the summer of 2005, when a project with detailed thunderstorm monitoring, including instruments to penetrate into clouds, gets under way.

Meanwhile, we don't have to mythologize about gods playing with lightning. We're doing it ourselves.