Detailed Footage Finally Reveals What Triggers Lightning

During a summer storm in 2018, a momentous lightning bolt flashed above a network of radio telescopes in the Netherlands. The telescopes’ detailed recordings, which were processed only recently, reveal something no one has seen before: lightning actually starting up inside a thundercloud.In a new paper that will soon be published in the journal Geophysical Research Letters, researchers used the observations to settle a long-standing debate about what triggers lightning—the first step in the mysterious process by which bolts arise, grow and propagate to the ground. “It’s kind of embarrassing. It’s the most energetic process on the planet, we have religions centered around this thing, and we have no idea how it works,” said Brian Hare, a lightning researcher at the University of Groningen and a co-author of the new paper.

The schoolbook picture is that, inside a thundercloud, hail falls as lighter ice crystals rise. The hail rubs off the ice crystals’ negatively charged electrons, leading the top of the cloud to become positively charged while the bottom becomes negatively charged. This creates an electric field that grows until a gigantic spark jumps across the sky.

Yet the electric fields inside clouds are about 10 times too weak to create sparks. “People have been sending balloons, rockets and airplanes into thunderstorms for decades and never seen electric fields anywhere near large enough,” said Joseph Dwyer, a physicist at the University of New Hampshire and a co-author on the new paper who has puzzled over the origins of lightning for over two decades. “It’s been a real mystery how this gets going.”A big impediment is that clouds are opaque; even the best cameras can’t peek inside to see the moment of initiation. Until recently, this left scientists little choice but to venture into the storm—something they’ve been trying since Benjamin Franklin’s famous kite experiment of 1752. (According to a contemporaneous account, Franklin attached a key to a kite and flew it beneath a thundercloud, observing that the kite became electrified.) More recently, weather balloons and rockets have offered snapshots of the interior, but their presence tends to interfere with the data by artificially creating sparks that wouldn’t naturally occur. “For a long time we really have not known what the conditions are inside a thunderstorm at the time and location that lightning initiates,” said Dwyer.

The opacity of stormclouds has until recently prevented scientists from seeing how lightning initiates.Photograph: George Rose/Getty Images