It’s hard to turn away from the dramatic spectacle of a summertime thunderstorm—brilliant flashes of pure electricity followed by a menacing rumble stirring in the clouds. But the typical lightning display pales in comparison with a baffling phenomenon that scientists have dubbed “superbolts”—lightning that can be up to 1,000 times more powerful than regular strikes. Now scientists offer a new potential explanation for how these extreme lightning bolts form.
Every second, up to 100 lightning discharges occur around the world. But superbolts are much stronger and rarer, making up about one thousandth of 1 percent of all lightning strikes. The phenomenon was first recorded in 1977 by a group of satellites tasked with detecting nuclear explosions. Researchers at the time observed that the flashes were 100 times more intense than ordinary lightning and lasted twice as long, about one millisecond.
A 2019 study found that superbolts are concentrated in three specific regions around the world—the North Atlantic Ocean, the Mediterranean Sea and the Altiplano in South America—and tend to peak from November to February. The same study suggested superbolts usually strike over water instead of land, the opposite of where ordinary lightning tends to appear. “We saw this paper and we started to think, ‘Why does this happen?’” says Avichay Efraim, a physicist at the Hebrew University of Jerusalem and lead author of the new study, which was published last month in the Journal of Geophysical Research: Atmospheres.
To investigate the phenomenon, the team matched strike data from the World-Wide Lightning Location Network (which monitors strikes using very low frequency, or VLF, radio receivers) with data on the properties of the storms that generated the lighting.
The explanation the researchers arrived at has to do with the internal mechanics of a thunderstorm. Inside turbulent storm clouds, the collision of tiny ice crystals and graupel—a kind of soft, frozen precipitation—creates an electric field, known as the charging zone, where lightning is born. Positively charged ice crystals are forced toward the top of the cloud by updrafts, currents of rising air. The negatively charged graupel are heavier, so they fall toward the bottom of the cloud. When the charges grow stronger, an electrostatic discharge eventually snaps through the air between them as a lightning bolt. (This also happens between the negatively charged lower part of the cloud and the positively charged ground.)
But superbolts, the team’s research suggests, happen when there is a shorter distance between the charging zone and Earth’s surface. “Our eyes were wide open,” Efraim says, describing the moment he saw the results. “It was very clear.”
That finding still didn’t describe a cause-and-effect relationship, however. To explain why distance might play a role, Efraim likens the phenomenon to a capacitor—an electronic component that stores energy in devices such as radios, projectors and refrigerators. “There are two charged plates and some material or air between them,” he explains. When these plates are too close, Efraim says, the electric field can get stronger and stronger, becoming more conductive, and if something similar happens in the clouds, stronger lightning results.
Efraim’s findings add to a body of research and multiple theories that attempt to explain the ultrahigh energy of the superbolts. One theory has suggested that large differences between the salinity of water and soil leads to higher energy in lightning. In the new paper, Efraim and his co-authors argue that this theory doesn’t explain why the different salinities of the Atlantic and the Mediterranean result in similar numbers of superbolts. Two other theories link aerosols from desert areas or sea spray to cloud invigoration and enhanced electrification. But these only explain the phenomena in specific regions, not globally.
Efraim says his team’s explanation applies to more locations where superbolts occur, but he and his colleagues couldn’t confirm that it explains the situation around the equator and the North Pacific. “There are a lot of open questions still left,” says Ningyu Liu, a professor of physics and astronomy at the University of New Hampshire, who was not involved in the new study. He points out that the study’s explanation works for superbolts occurring in the northeastern Atlantic Ocean and the Mediterranean Sea but not in other places. “Why are these two regions so different from other regions over water?” Liu asks.
Michael Peterson, an atmospheric scientist at Los Alamos National Laboratory, who was not involved in the new study, has suggested that the brightest superbolts viewed from space originate from positively charged cloud-to-ground electrostatic surges, compared with the negatively charged cloud-to-ground events that more frequently cause standard lightning strikes.
Peterson contends that the new study didn’t observe lightning that fits the current understanding of superbolts. In the 2019 study, the superbolts observed from the ground-based VLF radars—like the ones in Efraim and his colleagues’ more recent investigation—seemed to originate from mostly negatively charged cloud-to-ground events. Peterson says this earlier finding was a clue that the tools measuring different wavelengths of the electromagnetic spectrum were sensing a different kind of lightning. “So we’re dealing with a similar but, at the same time, a different set of lightning phenomenon,” he says.
Peterson would like to see the new results validated with electric field measurements, along with microphysical measurements from weather radars, to better understand how the behavior of the charged precipitation leads to lightning. With such measurements, “I’d be more likely to believe the theory, especially if there’s validation data available over the Andes, compared to over the midlatitude oceans,” he says. The types of storms that produce superbolts tend to happen over oceans, where the distance between the charging zone and the surface is greater than it is over mountain ranges. Peterson says this suggests physics—not height above Earth’s surface—might be the cause. Nevertheless, he adds that the new results are interesting and are a step forward in understanding the types of lightning physics involved.
In response to the critiques of the new paper’s conclusions, Efraim notes that “there have been many theories as to what causes these superbolts, and I think that this one is the strongest one.”
But he and his colleagues plan to dig into the problem more, specifically by focusing on superbolts around the equator. “We will have to go deeper into the data,” Efraim says, “and try to figure out what happens in the other regions that don’t necessarily follow our explanation.”