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Detailed footage finally reveals what triggers lightning

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  1. AugustusFerdinand

    Scientists have never been able to adequately explain where lightning comes from. Now the first detailed observations of its emergence inside a cloud have exposed how electric fields grow strong enough to let bolts fly.

    One idea holds that cosmic rays — particles from outer space — collide with electrons inside thunderstorms, triggering electron avalanches that strengthen the electric fields.

    The new observations point to the rival theory. It starts with clusters of ice crystals inside the cloud. Turbulent collisions between the needle-shaped crystals brush off some of their electrons, leaving one end of each ice crystal positively charged and the other negatively charged. The positive end draws electrons from nearby air molecules. More electrons flow in from air molecules that are farther away, forming ribbons of ionized air that extend from each ice crystal tip. These are called streamers.

    Each crystal tip gives rise to hordes of streamers, with individual streamers branching off again and again. The streamers heat the surrounding air, ripping electrons from air molecules en masse so that a larger current flows onto the ice crystals. Eventually a streamer becomes hot and conductive enough to turn into a leader — a channel along which a fully fledged streak of lightning can suddenly travel.

    “This is what we’re seeing,” said Christopher Sterpka, first author on the new paper. In a movie showing the initiation of the flash that the researchers made from the data, radio pulses grow exponentially, likely because of the deluge of streamers. “After the avalanche stops, we see a lightning leader nearby,” he said. In recent months, Sterpka has been compiling more lightning initiation movies that look similar to the first.

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