Can you imagine a lightning flash that spans the distance from Washington, D.C., to Boston?
The records set
The largest lightning discharge in recorded history occurred on Oct. 31, 2018, in southern Brazil, although the flash stretched from eastern Argentina all the way to the Atlantic. The complex, spider-like discharge spanned a horizontal distance of roughly 440 miles, the exact distance between Chicago and Toronto, or from Dallas to Memphis.
The WMO also reported an excessively long-duration lightning flash over Argentina on March 4, 2019. It lasted 16.73 seconds. That flash also spanned several hundred miles.
“These are mind-boggling incredible extremes,” Randy Cerveny, a professor at Arizona State University and a member of the WMO committee that certified the records, wrote in an email.
These were no ordinary lightning bolts; most strikes you see shooting out of the base of a thunderstorm are only a few miles long — although potentially more extensive if connecting to an intracloud discharge. And they usually last only a second or two.
The flashes over South America were in a league of their own: megaflashes. And there is reason to believe that megaflashes may not be as rare as once thought.
What is a ‘megaflash?’
For years, lightning has been treated as a local event. It result from an imbalance of electrical charge. When the buildup of a charge difference overwhelms the “dielectric strength” of air, a spark jumps between the two charges.
But emerging research reveals that some lightning events may be “mesoscale” in nature, reaching the scale of the occasionally massive sprawling storm complexes that create them.
Large-scale bands or arcs of thunderstorms, known as “mesoscale convective systems,” can yield extensive electric fields; once in a while, it’s possible for a disturbance in the field to trigger a lightning strike that will distribute charge over enormous sectors of that field. These so-called megaflashes do exactly that, sparks flying over hundreds of miles.
But brewing these overachieving bolts isn’t an everyday occurrence. To create a massive lightning bolt, you need an equally gargantuan storm. That’s where South America’s vicious summertime storm complexes enter the equation.
South America’s lightning factories: MCSs
Mesoscale convective systems, or MCSs, are a staple of the summertime in South America. An MCS is a thunderstorm complex that stretches about 100 kilometers or more across — or a little over 60 miles — but can grow much larger.
They form during the late afternoon and evening east of South America’s mountains, raging well into the night and frequently blossoming over areas up to 500 miles wide. Other prolific lightning-producing thunderstorms are common across the South American “Altiplano,” or the Andean Plateau, in parts of Bolivia and Peru.
On occasion, two MCSs can even merge over South America, sometimes amplifying their power and causing lightning-generating interactions between their electric fields. Colliding MCSs are unusual in the United States but are more common in South America due to more chaotic steering currents stemming from what are known as atmospheric “bore waves” in the lee of the Andes.
The depth of a lightning strike is limited by how tall a thunderstorm is, and lightning rarely jumps more than 10 or 15 miles into the “clear air” outside a storm. But when a thunderstorm covers more horizontal real estate, its region of electrical influence grows. And with more room to gallop, so, too, can the size of its lightning strikes.
Sometimes, a discharge in one part of the cloud will trigger an electrical disturbance elsewhere, which permits the lightning channel to continue growing. The South American storms investigated by the WMO took this to the extreme.
Megaflashes in the United States
Megaflashes aren’t unique to South America. In fact, we have them here in the United States, too — but only in recent years has technology become sufficiently supportive to allow atmospheric scientists the ability to map their electric tendrils.
The WMO noted that the recently verified records beat a previous record holder for long-distance flashes — a 200-mile-long discharge above Oklahoma on June 20, 2007.
An even more impressive discharge, which eventually spanned 300 miles but was not evaluated by the WMO, occurred in a similar area on the morning of Oct. 23, 2017. A thunderstorm raged near Thackerville, Okla., a little more than an hour’s drive north of the Dallas-Fort Worth Metroplex. A lightning strike illuminated skies near the Red River — Oklahoma’s southern border — at 12:13 a.m.
At the same time, the heavens were also ablaze near Burlington, Kan., as tendrils of spider lightning crawled eerily along the underbelly of a massive line of severe thunderstorms.
The two flashes — more than 300 miles apart — were connected to the same massive lightning bolt. The lightning strike was longer than the distance from Washington, D.C., to Hartford, Conn., and illuminated an area four times larger than the state of Connecticut.
The study of megaflashes is made possible by space-borne lightning detection systems onboard satellites. Both GOES satellites peering down on the United States feature a “geostationary lightning mapper,” or GLM. The satellite, some 22,236 miles up in space, is also able to resolve lightning discharges in South America.
“The GOES 16 sees the whole American continents, and adjacent oceans,” said Rachel Albrecht, a professor of meteorology at the University of São Palo in Brazil.
Scientists have heralded GOES 16, which was launched into orbit on Nov. 19, 2016, as a game changer for megaflash lightning detection.
“This dramatic augmentation of our space-based remote sensing capabilities has allowed the detection of previously unobserved extremes in lightning occurrence,” said Michael J. Peterson of the Space and Remote Sensing Group of Los Alamos National Laboratory in New Mexico.
Albrecht explained that, before the days of GOES 16, satellites could scan for lightning only in brief windows. That meant the vast majority of events were missed.
With greater spatial and temporal resolution, today’s satellites are able to remain on the constant lookout for megaflashes.
“We have measurements every two milliseconds with the [Global Lightning Mapper],” said Albrecht. “And we have all the chances to see those across the Americas, and also over the Pacific Ocean. [Europe] will launch their sensor in a couple of years, [and] China has a similar sensor.”
Cerveny hopes this new generation of satellites will open the doors to a greater understanding of these enormous lightning discharges.
“These are extraordinary records from single lightning flash events,” he said in a press release issued by the WMO on Wednesday. “It is likely that even greater extremes still exist, and that we will be able to observe them as lightning detection technology improves.”
The large distances lightning can travel underscore the importance of seeking shelter when thunderstorms approach, the WMO release stated. It advised that if lightning flashes and thunder sounds within 30 seconds to go indoors and then to wait at least 30 minutes after the last flash before venturing back out. This is known as the 30-30 rule.
According to the National Weather Service, lightning strikes in the United States about 25 million times a year and kills 20 or more people and injures hundreds. In 2020, four lightning deaths have been reported so far.
Jason Samenow contributed to this report.