Where there's smoke, there are sprites.

When slugger Mark McGwire started his home run streak this spring, meteorologists in the central United States were watching a different sort of hitting record in the making. From April through early June, a rare and powerful form of lightning blasted the ground far more frequently than normal, reports a team of scientists. At the same time, blood-colored flashes called sprites were lightning Earth's upper atmosphere at an unprecedented rate.

The meteorologists blame the unusual electric display on massive forest fires in Mexico that sent vast trails of smoke sailing over the United States.

"The amazing thing about this is that it shows the incredible interconnectedness of nature," says Walter A. Lyons of FMA Research in Fort Collins, Colo., whose team made the discovery. El Nino--warmed waters in the Pacific caused a drought in Mexico, which fed the fires that created the smoke that altered the lightning thousands of miles away and spawned sprites 50 kilometers up in the sky.

In normal thunderstorms, more than 90 percent of the lightning flashes hitting Earth carry negative charge from the clouds to the ground. The rest are called positive flashes because they have the opposite polarity.

This spring, a nationwide network of lightning detectors showed that storms over the central United States spawned positive flashes at three times their normal rate, Lyons and his colleagues report in the Oct. 2 Science. During a particularly intense series of storms in mid-May, 59 percent of the cloud-to-ground flashes were positive. Like McGwire, these storms were power hitters. The electric current in the positive flashes averaged twice the typical value of such lightning.

The scientists also detected an abnormal number of sprites--fleeting electric discharges that shoot upward, far above thunderstorms. One storm spawned 380 sprites; the most witnessed during studies of more than 100 storms over the past 6 years.

Satellite images linked the electric displays to the smoke. Storms with excess positive flashes developed in the band of smoke blowing north from Mexico.

Scientists have known for some time that forest fires can alter lightning, says Don J. Latham of the Intermountain Fire Sciences Laboratory in Missoula, Mont. In a study of one fire, Latham found that clouds above the conflagration generated mostly positive lightning. Fires, he suggests, generate such flashes because they send electrically charged particles into the sky.

The new study, however, shows that smoke can produce positive flashes thousands of kilometers from the fire, long after the electric charge in the smoke has dissipated. "It's an anomaly and it begs an explanation," says atmospheric physicist Charles B. Moore of the New Mexico Institute of Mining and Technology in Socorro.

The Mexican fires put many fine particles into the air, and that may have changed the way clouds became electrified, says Lyons. Because researchers still don't understand how clouds get charged up in thunderstorms, the recent discovery could provide a flash of insight into this process.