“It’s an incredibly strange phenomenon,” he said. “We only discovered them 10 years ago, when we could use a particular set of technologies to detect the start and end locations of the of lightning events.”
Megaflashes are not altogether uncommon, but they typically only occur in parts of the world where specific geographical and atmospheric conditions can produce the most severe thunderstorms, Cerveny said. In the Great Plains and across the Midwest, for instance, warm and humid air from the Gulf of Mexico collides with drier, colder air from the north, creating strong atmospheric instability.
When these conditions mix and produce severe storms, megaflashes of lightning can occur. These extra-long bolts of lightning have been observed before in the United States, Argentina and southern France, and scientists think they can also occur in parts of China and Australia, according to Cerveny.
The 2017 megaflash was produced by an immense storm that blanketed a huge swath of the U.S., from Texas up into Iowa and Missouri. Though megaflashes can extend across multiple states, they form high up in the atmosphere and so rarely cause damage on the ground, Cerveny said.
“They are upwards of 10,000 to 18,000 feet high, in the upper to middle layers of a thunderstorm,” he said.
The 515-mile-long lightning bolt was described in a study published Thursday in the Bulletin of the American Meteorological Society.
“These new findings highlight important public safety concerns about electrified clouds which can produce flashes which travel extremely large distances and have a major impact on the aviation sector and can spark wildfires,” WMO Secretary-General Celeste Saulo said in an accompanying statement.
The extreme conditions that spawn them are a reminder of how powerful and dangerous lightning storms can be. In the U.S., lightning kills roughly 20 people each year and injures hundreds more, according to the weather service.
With the classification Thursday, the 2017 lightning flash now surpasses the previous world record set five years ago by about 38 miles, according to the WMO. That bolt of lightning was unleashed April 29, 2020, and spanned 477.2 miles across parts of the southern U.S.
The 2017 megaflash was identified after scientists re-examined archival measurements taken when the storm occurred.
“When the original studies were done, we didn’t have the technology that we have today,” Cerveny said. “Now we have this instrument on a weather satellite that very accurately detects lightning and can precisely pinpoint where, how far and how long a lightning flash event takes place.”
Experts said it’s likely there will be even longer megaflashes found in the coming years, particularly as satellite technologies improve the ability to detect them.
“Over time, as the data record continues to expand, we will be able to observe even the rarest types of extreme lightning on Earth and investigate the broad impacts of lightning on society,” study lead author Michael Peterson, an atmospheric scientist in the Severe Storms Research Center at the Georgia Institute of Technology, said in a statement.
The WMO’s Committee on Weather and Climate Extremes keeps official records of global, hemispheric and regional extremes, including for temperature, rainfall, wind, hail, lightning, tornadoes and tropical cyclones.