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‘Superbolts’ of lightning strike when scientists least expect

Jagged streaks of lightning ripping across the sky are common to summer storms all over the world—that’s the season when the most strikes occur. But the biggest bolts, according to new research, strike most often from November to February–winter in the Northern Hemisphere. These rare “superbolts,” which are 1000 times stronger than the average strike, also seem to hit far more frequently over ocean than land.

To find out where and when this super-lightning occurs, researchers used a network of roughly 100 lightning detecting stations on six continents. The stations can “listen” for lightning-produced radio waves by tuning in to the very low-frequency range of 5 to 18 kilohertz. From 2010 to 2018, the team recorded 2 billion lightning strikes.

To figure out whether a bolt was “super,” the researchers compared the time it took radio waves from a given strike to reach seven or more stations. This let them triangulate the lightning’s location and power—a discharge strength of more than 1 million joules within the narrow bandwidth they measured meant that a strike was a “superbolt.”

If 1 million joules sounds paltry for a superbolt it’s because the detection stations were only measuring the tiny slice of the lightning’s energy that fell between 5 and 18 kilohertz. This means they didn’t measure the total electrical power of a superbolt, but researchers say their results appear consistent with prior work suggesting these big bolts range between 10 billion and 1 trillion watts of electrical power. At the upper limit that’s enough to power the average U.S. home for nearly 1 month.

Out of the haze of ozone and rumbling thunder, the researchers recorded more than 8000 superbolts, they report today in the Journal of Geophysical Research: Atmospheres. Most happened between November and February, and most hit the ocean—which is surprising because some 90% of all lightning strikes land. The superbolts clustered in the Mediterranean Sea, the northeast Atlantic Ocean, and the Andes mountain range. 

Scientists still aren’t sure what causes superbolts, which make up less than one-thousandth of a percent of all global strikes. Clues may lie in the frequency with which they strike: The most superbolts ever recorded were in late 2013, followed closely by 2014. For now, the researchers are turning to sunspots and cosmic rays for an explanation. If that doesn’t pan out, stay tuned.

*Correction, 11 September, 11:30 a.m.: This story has been corrected to reflect the fact that 1 billion joules was just a fraction of the total energy of the superbolt.