On October 22, 2017, a thunderstorm unfurled across the Great Plains of the United States. To anyone watching from the ground, it may have looked like just another thunderstorm. But far above, beyond clouds and human perception, something extraordinary happened: a lightning bolt arced across the sky from east Texas all the way to Missouri, traveling a staggering 829 kilometers (515 miles).

It wasn’t until years later, after scientists combed through satellite data, that the true scale of the event came to light. The World Meteorological Organization (WMO) has now confirmed it as the longest lightning flash ever recorded — a single electrical discharge that stretched farther than the distance from Paris to Venice.

“We call it megaflash lightning and we’re just now figuring out the mechanics of how and why it occurs,” said Randy Cerveny, a geographical scientist at Arizona State University and a contributor to the study. “It is likely that even greater extremes still exist.”

The bolt surpassed the previous record, a 768-kilometer (477 miles) flash from April 2020, also over the Great Plains. Both were part of a class of rare atmospheric phenomena known as megaflashes — lightning events that extend more than 100 kilometers (62 miles) through thunderclouds.

What Makes a Megaflash?

Unlike the classic forked bolts that crash to Earth in a matter of milliseconds, megaflashes slither horizontally through massive storm systems, rarely touching the ground. Most lightning bolts are short (typically under 20 kilometers) and just an inch wide. But megaflashes travel across entire states.

So how do we even know one happened?

Before 2016, researchers relied on ground-based radio sensors to track lightning. These systems struggled to capture the full scale of long, complex flashes. “The extremes of what lightning is capable of is difficult to study because it pushes the boundaries of what we can practically observe,” said Michael Peterson, lead author of the study and a researcher at Georgia Tech’s Severe Storms Research Center.

That changed with the launch of the GOES-16 satellite, the first of a new generation of geostationary weather satellites operated by the National Oceanic and Atmospheric Administration (NOAA). GOES-16 carries a Geostationary Lightning Mapper, a high-resolution sensor capable of detecting and tracking millions of lightning bolts per day from orbit.

“Adding continuous measurements from geostationary orbit was a major advance,” Peterson explained. “We are now at a point where most of the global megaflash hotspots are covered by a geostationary satellite.”

It took careful analysis to confirm that the Texas-to-Missouri bolt was a single connected flash and not multiple strikes happening in rapid sequence. Scientists painstakingly reconstructed the flash in three dimensions using satellite data, mapping both positively charged (red) and negatively charged (blue) branches across a composite image of the sky.

This bolt, it turns out, had stitched itself through nearly the entire cloud complex — a colossal atmospheric structure the size of New Jersey, churning for at least 14 hours.

Why the Great Plains?

Both the 2017 and 2020 record-breaking lightning flashes formed over the same region because powerful thunderstorm systems, called mesoscale convective systems, frequently develop across the Great Plains. These massive storms, often covering vast areas, provide the perfect conditions for megaflashes. According to Peterson, less than 1% of thunderstorms globally are capable of producing megaflashes — but in this region, the odds are much higher.

There’s more to this insight than mere meteorological trivia. There are safety implications, too. “[The] everyday implications of these events are that lightning can travel quite long distances away from the parent thunderstorm,” said Prof. Cerveny, who also serves as a rapporteur on climate extremes for the WMO, as per The Guardian. “Because of this fact, people need to limit their outdoor activities during lightning-producing thunderstorms.”

The WMO advises that during a lightning storm, the safest places to take shelter are in substantial buildings with wiring and plumbing, or in fully enclosed metal-topped vehicles.

And while the 829-kilometer flash was stunning in length, the GOES satellites have also helped identify the longest-lasting single lightning bolt: a 17.1-second flash bolted over the skies over Uruguay and northern Argentina in June 2020.

What Scientists Hope to Catch Next

As satellite coverage expands and data-processing techniques improve, scientists believe we’re likely to find even longer and more extreme lightning events. “It is likely that even greater extremes still exist,” Cerveny noted. “We will be able to observe even the rarest types of extreme lightning on Earth and investigate the broad impacts of lightning on society.”

Lightning influences forest fires, atmospheric chemistry, and even — according to some theories — the origins of life on Earth. But we’re only beginning to understand its full range.