Researchers have found a new way to estimate the power of lightning strikes. Rather than estimating the electrical activity in the air, the technique looks at the impact when a bolt of lightning strikes sand, forming what is known as a fulgurite.

“Everyone knows there is a lot of energy in a lightning bolt, but how much?” said Dr Matthew Pasek of the University of South Florida in a statement. The answer has proven hard to obtain. 

Coming from Florida, possibly the most lightning-prone state in the US, Pasek was confronted with the question often. Locations where lightning touches sandy ground, rather than hitting a tree or building, are also common there.

“When lightning strikes the sand, it may generate a cylindrical tube of glass called a fulgurite,” Pasek explained. “The structure of the fulgurite, created by the energy and heat in a lightning strike, can tell us a lot about the nature of the strike, particularly about the amount of energy in a single bolt of lightning.”

^{Fulgurites collected from the study site. Differences in color reflect the thickness of the glass wall. Pasek & Hurst/Scientific Reports}

Polk County sand mines contain hundreds of fulgurites, some dating back thousands of years. Pasek and colleagues collected 266 for analysis and published their results in Scientific Reports. The longest fulgurite ever recorded was 5 meters (17 feet) long, but the average Pasek collected was a rather more humble 5 centimeters (2 inches).

Estimating energy from the fulgurite a lightning strike leaves behind is no simple matter. The shape of “fulgurites is strongly dependent on material composition, and for natural fulgurites, the thickness of the glass is inversely related to fraction of material composed of SiO₂," the paper observes. Moreover, some older fulgurites have broken since formation, and putting together the pieces is not always easy. However, once such factors are accounted for, fulgurite size provides an indication of the energy that made it.

The authors report that typical strikes on quartz sand have an energy around 1.4 Megajoules per meter of fulgurite produced. This means that a typical fulgurite dissipates less than 1 percent of the energy in the lightning. Nevertheless, the paper argues that by calculating the amount of energy released in making a fulgurite it should be possible to estimate the total energy in the strike.

The findings are consistent with observations through other means that lightning bolts can have temperatures that exceed 30,000 K (53,000 ºF), and potential differences of 100 million volts.

One of Peake’s findings is that lightning follows a lognormal trend in the distribution of energy in bolts. A lognormal trend means that the largest or most powerful examples of a phenomenon happen more frequently than would occur under the Gaussian distribution that is more common for natural phenomena. “This means that the big lightning strikes are really big,” Pasek said. 

 ^{Close up of one of the fulgurite. Pasek & Hurst/Scientific Reports}