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Salt Spray May Stifle Lightning over the Sea

2022-11-08 12:08:01

Most lightning occurs over land, but most precipitation occurs over the ocean, said Daniel Rosenfeld (Hebrew Univ), Robert Holzworth (ESS) and other coauthors, of a new study in Nature Communications (https://www.nature.com/articles/s41467-022-31714-5 ). For the past 20-some years, researchers have been batting about ideas on why. “I had many friendly arguments with my colleagues,” Rosenfeld said. Over land and sea, lightning forms from the collisions of ice particles and pellets in a cloud. The process starts with small water droplets being wafted high into a cloud, where it’s cold. Even when the temperature is below freezing, the droplets can stay liquid, becoming supercooled. But eventually, some droplets form ice crystals, and when other supercooled droplets ram into them, these droplets freeze fast. These pellets, called graupel, fall under their own weight. When graupel collides with small ice crystals, the collisions create electrical charges. The falling graupel tends to grab the negative charges, whereas rising ice crystals tend to stash positive ones. Eventually, the air between those separated charges breaks down, sending a bolt of lightning crashing through the cloud. But sea spray forms large salt particles—greater than 1 micrometer. Water gloms onto these aerosols, creating large raindrops. The falling rain starves oceanic clouds of the water needed to create zaps of lightning, researchers reported. WWLLN lightning data were used to make the case about lightning.

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