On 29 April 2020, a single bolt of lightning ignited above eastern Texas, leapt across the Louisiana border, and stopped only after it had crossed Mississippi — 768 kilometres of continuous electrical discharge, certified by the World Meteorological Organization as the longest single flash recorded at that time (now the second-longest ever measured). End to end, it covered roughly the straight-line distance between New York City and Columbus, Ohio. It did so in under seven seconds.
The bolt did not strike the ground along most of its path. It travelled horizontally, threaded through the underside of a sprawling overnight thunderstorm complex, branching as it went, illuminating a strip of sky longer than the United Kingdom.
What a megaflash actually is
Most lightning is short. The average cloud-to-ground bolt covers less than 16 kilometres. A flash that crosses 100 kilometres of sky earns a separate name: megaflash. According to research studying record-setting flashes, fewer than 1 percent of thunderstorms produce them at all, and only a fraction of those exceed 500 kilometres.
The April 2020 flash was unusual even among megaflashes. It was photographed from orbit by the Geostationary Lightning Mapper, an instrument bolted to the GOES-16 satellite that scans the western hemisphere for optical lightning emissions 500 times a second. Without that satellite, no ground-based network could have captured the full extent of it. The flash was simply too long for any single sensor on Earth to see.
Texas, Louisiana, Mississippi — in one breath
The storm system that produced the bolt was a mesoscale convective complex, the kind of overnight thunderstorm cluster that drifts across the Gulf states each spring. These systems can stretch 1,000 kilometres across and persist for twelve hours or more. Inside them, sheets of charge accumulate along the stratiform region. This is the broad, low-rainfall back end of the storm, where ice crystals settle into thin horizontal layers.
That layered geometry is what makes megaflashes possible. A bolt that begins as a vertical strike near the storm’s leading edge can find a low-resistance pathway running rearward through the stratiform deck, and follow it for hundreds of kilometres before the charge difference dissipates. The 2020 flash did exactly that, beginning over Texas and propagating east-northeast across two state lines before terminating in Mississippi.
At the speed of a typical lightning leader, somewhere between 100 and 300 kilometres per second for the dart leader phase, covering 768 kilometres in under seven seconds matches the physics. A car driving the same route on Interstate 20 would need about eight hours.
What scientists finally figured out about the trigger
For decades, the question of what physically initiates a lightning bolt inside a thundercloud was open. The electric fields measured inside storms, around 100 to 400 kilovolts per metre, are too weak by an order of magnitude to ionise air directly. Something else has to start the cascade.
In 2025, a team led by Victor Pasko at the Penn State School of Electrical Engineering and Computer Science published the first quantitative model of lightning initiation. The trigger, they showed, is a chain reaction seeded by cosmic rays.
A high-energy particle from deep space enters the atmosphere, collides with an air molecule, and produces a shower of relativistic electrons. Inside the strong electric field of a thundercloud, those electrons accelerate further, crashing into nitrogen and oxygen molecules. The collisions emit X-rays. Those X-rays free more electrons through the photoelectric effect. The new electrons accelerate, emit more X-rays, free more electrons. The avalanche compounds in a fraction of a millisecond.
According to the research, the findings provide the first precise, quantitative explanation for how lightning initiates in nature, connecting X-rays, electric fields, and electron avalanche physics.
The X-ray burst that accompanies this process is called a terrestrial gamma-ray flash. They last between 0.2 and 3.5 milliseconds and reach energies up to 20 million electronvolts. That is roughly a million times more energetic than visible light. They are invisible to the naked eye. Pilots flying through thunderstorms have been measured passing through them without seeing anything at all.
Why the April 2020 flash was a record, not an outlier
The 768-kilometre figure was not announced until February 2022, when the World Meteorological Organization’s committee on weather and climate extremes finished verifying the satellite data. The previous record, a 709-kilometre flash across southern Brazil in October 2018, was beaten by about 60 kilometres.
Both records depended on the same instrument: the Geostationary Lightning Mapper. Before it came online in 2017, lightning detection relied on ground networks that triangulated radio emissions from individual strokes. Those networks were never designed to stitch together a single flash spanning three states. The megaflash record has been broken several times since the satellite started watching, not because storms are getting bigger, but because humans finally have the right eyes for them.
In July 2025, that record fell again, to a megaflash that had actually occurred years earlier, in October 2017. A re-examination of GOES-16 satellite data confirmed an 829-kilometre flash, about 515 miles, across the Great Plains from eastern Texas almost to Kansas City. Researchers refer to these record-breaking events as megaflash lightning. The 2020 Texas-to-Mississippi bolt is no longer the longest. It is still the second longest ever measured.
The danger of a clear sky
A megaflash can deliver a ground strike far from any visible storm cloud. A flash like the one that crossed three states in April 2020 can drop contacts far along its path, some of them under skies that observers on the ground would have described as overcast at worst, no rain, no thunder audible yet.
The standard safety guidance, if you can hear thunder, you can be struck, assumes a flash of conventional length. A bolt threading 700 kilometres of stratiform cloud can drop a stroke 50 or 60 kilometres from any rain, well beyond the range at which thunder is audible. The National Weather Service calls these strikes “bolts from the blue,” and they account for a disproportionate share of lightning fatalities.
The 2020 megaflash, fortunately, caused no known casualties. The 2018 Brazilian record-holder also produced no recorded casualties. The danger is statistical. If megaflashes become more frequent, and there is no evidence yet that they are, only that we are now able to count them, the population of people standing under apparently safe skies during distant storms will rise with them.
What lightning sounds like from 100 kilometres away
It sounds like nothing. Thunder travels at roughly 343 metres per second through warm air, and is absorbed and refracted by atmospheric layers within about 25 kilometres of its source. A bolt 50 kilometres away produces no audible sound at the surface. A bolt 700 kilometres away might as well be on the moon.
What it looks like, if you happen to be standing under the storm’s stratiform deck, is a slow horizontal flicker that crosses the entire visible sky, persists for several seconds, and disappears. It does not crack. It does not boom. The branches drift sideways across the cloud base the way a river finds its way across a floodplain, except that the river is moving at roughly half the speed of light, and the floodplain is the underside of a thunderstorm the size of a small country.
The numbers, in closing
Since the Geostationary Lightning Mapper came online in 2017, three separate flashes longer than 700 kilometres have been certified by the World Meteorological Organization: the 2018 Brazilian bolt at 709 kilometres, the 2020 Texas-to-Mississippi flash at 768 kilometres, and the re-examined 2017 Great Plains flash at 829 kilometres. All three were detected from orbit. None would have been measurable end to end by any ground network in operation at the time.
The detection rate has shifted accordingly. Before 2017, the longest verified flash on record stood at 321 kilometres, set in Oklahoma in 2007. Within eight years of continuous satellite coverage, that figure has more than doubled. The instrument samples the western hemisphere 500 times a second at a spatial resolution of roughly 8 kilometres per pixel, and it is currently the boundary condition on how long a flash humans can confirm. Anything longer than 829 kilometres is, for now, only theoretical.
