Mexican Fires Charge Distant Clouds

The ancient Greeks believed that lightning bolts sprang from the rage of Zeus in his home on Olympus. Now an odd new discovery suggests that Zeus' moods have a long reach indeed: Last spring, smoke from massive fires in Mexico spawned strong, sustained lightning over the Great Plains, thousands of kilometers away. The number of positively charged bolts tripled and carried twice as much current as similar flashes produced by smoke-free storms, scientists report in tomorrow's issue of Science.

The amped-up storms struck between 8 April and 7 June, when a pall of smoke from drought-related fires in southern Mexico drifted north and dirtied the air from Texas to Canada, especially over the southern Great Plains. Atmospheric scientist Walter Lyons and his colleagues at FMA Research in Fort Collins, Colorado, noticed unusual behavior in storms over the plains: They triggered huge numbers of "sprites"--ghostly red glows of charged nitrogen molecules often seen high above violent storms.

The team then checked data from the National Lightning Detection Network, which tracks the location, charge, and strength of most bolts by monitoring the bursts of radio static they produce. The data revealed that the lightning from smoke-enriched storms was stronger. "There were an abnormal number of positive cloud-to-ground flashes, and the peak currents [of those flashes] doubled," Lyons says. "That really startled us." Positive lightning usually accounts for 10% of all bolts but produces a sustained pulse and can do more damage than negative lightning. "This is a wake-up call for those who thought they understood thunderstorm electrification," says atmospheric scientist Andrew Detwiler of the South Dakota School of Mines and Technology in Rapid City.

Lyons's team suspects that the extra positive charge may arise because tiny smoke particles provide more nuclei around which cloud droplets condense. That makes the droplets smaller, which in turn may alter how they acquire electrical charge when they freeze into ice grains and are churned high in the thunderstorms. Clearer answers could come from airplane studies of smoky thunderstorms, says Lawrence Radke of the National Center for Atmospheric Research in Boulder, Colorado. "It would be thrilling to have observations in these clouds to see what actually changes," he says. That will take another season of fires. In the meantime, "it's all speculation."