NASA-funded scientists have recently learned that cloud-to-ground lightning frequently strikes the ground in two or more places and that the chances of being struck are about 45 percent higher than what people commonly assume.
Initial Leader Forks and Strikes the Ground in Two Places - Within Valine and Krider's study sample of 386 flashes, 136 flashes (35%) struck the ground in two or more places that were separated by tens of meters (yards) or more, such as the strike pictured here. The 386 flashes produced a total of 558 different strike points; therefore, on average, each cloud-to-ground flash struck the ground in 1.45 places.
Recently, William C. Valine and E. Philip Krider in the Institute of Atmospheric Physics at the University of Arizona, co-authors of the study, took to the field using video and other technology to study lightning, which is one of the biggest weather-related killers in the United States, superseded only by extreme heat and flooding.
They recorded 386 cloud-to-ground (CG) lightning flashes on videotape during the summer of 1997 in Tucson, Arizona. They found that within their sample of 386 flashes, 136 flashes (35 percent) struck the ground in two or more places that were separated by tens of meters (yards) or more. There were a total of 558 different strike points; therefore, on average, each cloud-to-ground flash struck the ground in 1.45 places.
"Most people assume that lightning strikes in only one place. In this research, we've documented that lightning definitely strikes more than one place about a third of the time," Krider said. "If you want to quantify the chances of being struck by lightning, they are about 45 percent higher than the number of flashes because, on average, there are about 1.45 strike points per CG flash."
Within that group of 136 flashes, termed "multiple channel flashes," 88 had two or more separate and distinct channels (or paths) between the cloud base and the ground. Thirty-seven of the flashes forked below the cloud base and struck ground in two or more places. Eleven flashes exhibited both types of behavior. In other words, during the observations in Arizona, for every fork below the cloud there were approximately twice as many flashes that had separate and distinct paths, a ratio that is consistent with previous measurements in Florida.
Valine and Krider also confirmed that after an initial stroke, 67 percent of the new strike points were produced by the second stroke in the flash, rather than the third or fourth stroke. In other words, if any subsequent stroke is going to strike a place different from the first stroke, it is usually the second stroke that does so. The third and fourth strokes usually follow the same path as the second stroke.
Lightning occurs when there is a discharge of electricity between large volumes of excess positive and negative charge that accumulate in thunder clouds. Lightning most commonly occurs in thunderstorms, but it also can occur in snowstorms, sandstorms, in the ejected material over volcanoes. Most lightning takes place within or between clouds; on average, only about one-third of all discharges actually strike the ground. The peak temperature in a lightning channel is around 60,000 degrees Fahrenheit, or about 5 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 a safe distance from a previous flash is at least 10 to 13 km (6 to 8 miles) as opposed to the 3 to 5 km (2-3 miles) that experts had previously advised.