1. Most Lightning Is Shorter Than 10 Kilometers, So 829 Is Enormous
A lightning flash can run between a cloud and the ground, or stay inside a cloud. Either way, it is typically shorter than 10 kilometers [3]. That is about the distance of a long bike ride.
The record flash was about 83 times that long. Scientists measured it at 829 kilometers, with an uncertainty of 8 kilometers, from eastern Texas to near Kansas City. The World Meteorological Organization (WMO), the United Nations weather agency, compares this with the distance from Paris to Venice. A car would need about eight to nine hours to drive it, and a passenger jet at least 90 minutes to fly it [1].
Scientists call flashes of this kind "megaflashes": lightning that stretches hundreds of kilometers sideways across the sky. The 829 kilometers is the length of one flash spreading through the sky, not a single bolt that hit the ground. According to Physics Today, the flash lasted about 7.4 seconds [3].
2. The Record Has Been Broken Again and Again
For a long time, scientists thought lightning could hardly travel more than 100 kilometers sideways [3]. Then the records kept growing. In 2007, a ground network in Oklahoma recorded a flash 321 kilometers long. In 2018, a flash over southern Brazil covered 709 kilometers, more than double that, about the distance from Boston to Washington, D.C. [2]. A flash over the southern United States on April 29, 2020 reached 768 kilometers [1].
The 829-kilometer flash beat that by 61 kilometers. Oddly, it is the oldest of the three big ones: it happened in 2017, before the Brazilian flash, but the WMO certified it only in 2025 [1]. Why so late? The answer has to do with how such a flash is measured, which we come to in section 4.
3. A Storm That Spreads Out Gives Lightning Room to Run
First, how does ordinary lightning start? Inside a tall storm cloud, soft hail called graupel collides with ice particles. The graupel picks up negative charge, and lighter, positively charged ice drifts toward the top. When the electric field grows strong enough, the air gives way and lightning flashes [3]. These charged regions are usually only a few kilometers across, and in the biggest storms tens of kilometers.
Megaflashes need something larger. Thunderstorms can organize into huge groups called mesoscale convective systems. One kind has a long front of towering, electrically active clouds, followed by a wide area of shallower, layered cloud. In the warmer months, such systems cross the central United States and can last more than 12 hours [3].
The shallow cloud behind the front is far from the strong updrafts, yet it holds layers of weak electric charge. Some of that charge forms there. Some is carried into the trailing cloud by positively charged ice crystals, which drift backward and downward from the tops of distant towers. According to the researchers writing in Physics Today, these pockets of charge can link up into what they call a "cloudy capacitor", big enough to cover several states. That makes an ideal setting for a megaflash [3].
A simple way to picture it is a long row of dominoes. One push can tip the whole line. In the same way, a flash that finds weakly charged cloud stretching for hundreds of kilometers can keep going. This is only a picture to help you think about it. Real lightning does not move in neat order, and scientists are still studying megaflashes. In the Physics Today authors' words, a megaflash is just "a much, much bigger spark than usual" [3].
4. Satellites Can Measure Flashes That Ground Networks Cannot
To know how long a flash is, you have to see all of it. The first megaflash records came from ground networks called lightning mapping arrays: radio receivers spread around an area about the size of a city. Like weather radar, they only work within line of sight, so there is a limit to how long a flash they can follow [3]. The WMO notes that many lightning scientists knew about this limit [2].
One way around this limit was to watch from space. NOAA's GOES-16 satellite, launched in 2016, carries the Geostationary Lightning Mapper. A geostationary satellite appears to stay above the same spot on Earth, so it can watch a huge area all the time. Its fast optical detectors report where and when lightning flashes happen across the Americas. Its view includes two known megaflash hot spots, the Great Plains of North America and the La Plata Basin in South America [3]. Europe and China have launched similar instruments [2].
5. Why a 2017 Flash Was Only Certified as a Record in 2025
A satellite does not record "one long flash". It records flashes of light, each with a place and a time. The software that grouped these flashes was not designed for flashes that last seconds and spread across several states. So the record flash was not picked out in the original analysis of the 2017 storm. It showed up later, when scientists re-examined the storm data [1][3].
The WMO's record keepers, who check weather extremes, then certified it in 2025. They used the same method as for earlier records: the greatest distance in a straight line across the Earth's curved surface between the farthest ends of the flash [1].
The story suggests that even longer flashes may already be hiding in old data. WMO rapporteur Randall Cerveny said it is likely that even greater extremes exist, and that more will be seen as good lightning measurements pile up [1].
6. Why Very Long Flashes Matter for Staying Safe
Studying how far lightning can reach is also about safety. The researchers explain that one megaflash can produce a hundred or more strikes on the ground, some tens to hundreds of kilometers away from the busiest part of the storm. The record flash had 116 ground strikes linked to it [3]. Fortunately, no deaths from that thunderstorm were reported [3].
Such strikes are a bit like "bolts from the blue", which leave a thundercloud and hit the ground in clear air up to about 15 kilometers away. A megaflash strike may land far from the storm's busiest area. Some also carry very strong or long-lasting currents, which can start fires [3].
The advice from NOAA and the National Lightning Safety Council still works well: "When thunder roars, go indoors," and wait 30 minutes after the last rumble before going back outside [3][2]. Researchers say only a few rare situations, such as big outdoor crowds, launches, or power systems, may need extra care [3].
7. Measure 829 Kilometers on a Map Yourself
You can feel how long this flash was without going near a storm. You need a map or atlas with a scale bar and a piece of string. Keep a pencil handy to mark the string.
First, lay the string along the scale bar for 100 kilometers (or 100 miles, if that is what your map shows) and mark it. Now step that length along the map from your home, eight times, to reach about 800 kilometers. Where does the string end up?
Then try 10 kilometers on the same scale. It will be a tiny piece of string. Putting the two side by side shows the gap between an ordinary flash and the record better than the numbers do.
If you want to dig deeper, read the WMO announcement from 2025, which explains the record and how it was found, and the Physics Today article, which is written for general readers and goes further into how the storms work. Both are listed below. What else might hide in old satellite data?