Lightning Really Does Strike More Than Twice

A dubious old saw says that lightning doesn’t strike the same place twice. Now new research shows just how frequently lightning strikes the ground in multiple places.

University of Arizona atmospheric scientists William C. Valine and E. Philip Krider have recently confirmed that cloud-to-ground lightning commonly strikes the ground in two or more places. The finding means that the chances of being struck are about 45 percent higher than people generally assume.

Lightning is one of the biggest weather-related killers in the United States, topped only by extreme heat and flooding.

Valine and Krider of the UA Institute of Atmospheric Physics used two VHS video camera/recorder systems operating simultaneously atop a 5-story building on the UA campus in Tucson, Ariz., in this study. Valine analyzed the video tapes frame-by-frame for a precise understanding of the sequence and geometry of the strokes that make up a lightning flash.

They recorded 386 cloud-to-ground lightning flashes on videotape during the summer of 1997. They found that 136 of these 386 flashes, or 35 percent, struck the ground in two or more places that were separated by tens of meters (yards) or more. The 386 flashes hit the ground in 558 different places, so on average, each cloud-to-ground flash struck the ground in 1.45 places.

“Most people assume that lightning strikes in only one place,” Professor Krider said. “In this research, we’ve documented that lightning definitely strikes in two or more places about a third of the time. If you want to quantify the chances of being struck by lightning, they are about 45 percent higher than the number of flashes because each flash strikes 1.45 places, on average.”

The average number of strike points per cloud-to-ground flash “is remarkably consistent with the values found in France and other geographical regions,” the UA scientists said. Further, the “flash counts” of cloud-to-ground lightning provided by the U.S. National Lightning Detection Network and all other such lightning detection systems should be multiplied by about 1.5 to estimate the true number of strike points, they added.
Composite image (left) of the two images (right) that shows an “altered channel flash” that forked and then struck the ground at two places. (Video photos: Michael Garay)

Within the 136 so-called “multiple channel flashes,” 88 were “new channel flashes” — they had two or more separate and distinct channels, or paths, between the cloud base and the ground. Thirty-seven of the flashes were “altered channel flashes” — they forked below the cloud and struck the ground at two or more places. The remaining 11 flashes combined both these behaviors -? they produced both new and altered channel flashes. That is, every fork below the cloud discharged two or more separate flashes.

The height of the cloud base above the local terrain is a factor in determining how lightning attaches to the ground, Krider noted. Valine re-analyzed the entire Arizona data set substituting Florida’s one kilometer (3/5 mile) cloud-base height for Arizona’s 3 kilometer (1 and 4/5 mile) cloud-base height. The ratio of lightning types in the Arizona study proved to be consistent with previous measurements made in Florida.

Valine and Krider also confirmed that after an initial strike, 67 percent of the new strike points were produced by the second stroke in the flash, rather than by the third or fourth stroke. The third and fourth strokes usually follow the same path as the second stroke.

Lightning is a large discharge of electricity between accumulations of positive and negative charges in thunder clouds. Lightning most commonly occurs in thunderstorms, but it also can occur in snowstorms, sandstorms, and in ejecta blown up from volcanoes. Most lightning occurs within or between clouds, with only about a third of all discharges actually striking ground. The peak temperature in a lightning channel is about 60,000 degrees Fahrenheit, or about five times hotter than the surface of the sun.

According to the National Weather Service, lightning causes an average of 93 deaths and 300 injuries in the United States each year. The National Severe Storms Laboratory recommends that 6 to 8 miles (10 to 13 kilometers) is a safe distance from a previous flash. Experts had previously advised that 2 to 3 miles (3 to 5 kilometers) was safe.

Valine and Krider reported their research in the latest print issue of the Journal of Geophysical Research, an American Geophysical Union publication, in the article, “Statistics and characteristics of cloud-to-ground lightning with multiple ground contacts.” Publication of this work has been supported by NASA.

More information and images are available from the NASA Goddard Space Flight Center web site, http://www.gsfc.nasa.gov

Download a PowerPoint presentation that shows the lightning types at http://ali.opi.Arizona.edu/silk/downloads/Examples.ppt — BUT NOTE that this is an 8 MB file.