The single flash extended 767.9km across Texas, Louisiana and Mississippi in April 2020, the World Meteorological Organisation said onMonday.
That beat the old record set in 2018 in Brazil of 709km.
Also in 2020, a single lightning flash over Uruguay and northern Argentina lasted 17.1 seconds, nipping the old time record of 16.7 seconds.
Normally lightning doesn't stretch farther than 15km and lasts less than a second, said Arizona State University's Randall Cerveny, who is the chief of records confirmation for the meteorological organisation.
"These two lightning flash records are absolutely extraordinary," Cerveny said in an email.
Both were cloud-to-cloud, several thousand feet above ground level, so no one was in danger, he said.
These records, which are not linked to climate change, were spotted and confirmed thanks to new satellite tracking technology.
Both regions are two of the few places in the world prone to the type of intense storms that can produce what are called "megaflashes, " Cerveny said.
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