When it comes to lightning, don’t pass the salt

One of nature’s most intense spectacles can be tamed with humble sea salt, the same stuff that graces some dining tables. Researchers recently found that flash frequency decreases by up to 90 percent in the presence of saline spray. That makes sense based on how electric fields build up in clouds, the team suggests. These new findings, published this month in the journal Nature Communications, help explain why thunderstorms occur much less frequently over seas than over land.

From rogue waves to floating debris, seafarers have much to fear. But at least lightning strikes in general are not on the list: compared to the lightning recorded over the continents, only about a tenth as many strikes occur at sea. Exactly why remained a mystery for a long time. And that’s a “glaring flaw” in our understanding, said David Romps, a physicist at the University of California, Berkeley who is not involved with the research.

To get to the bottom of this mystery, Zengxin Pan, an atmospheric scientist at the Hebrew University of Jerusalem, and his colleagues analyzed data from more than 75,000 recent thunderstorms. The researchers combined observations from the World Wide Lightning Location Network, a global archive of high-frequency observations of lightning, with satellite-based measurements of cloud properties, precipitation and tiny aerosol particles in the air. The goal was to track how convective clouds — the birthplaces of lightning — evolved over time under different atmospheric conditions, said Daniel Rosenfeld, a co-author on the study, who is also an atmospheric scientist at the Hebrew University of Jerusalem.

The researchers first showed that aerosols smaller than a few millionths of an inch — like dust and soot particles associated with air pollution — tend to increase flash frequency for a given amount of rainfall. That was to be expected, said Dr. Rose field. Fine aerosols function like miniature landing pads for water in clouds. Liquid tends to stick to them and form droplets, but these droplets are small enough that they tend to remain in a cloud rather than falling to the ground as raindrops, Dr. Rose field.

And because the presence of water in clouds creates the intense electric fields that lead to lightning, dirty air leads to more lightning. (This effect has been previously demonstrated in oceanic shipping lanes navigated by polluting ships.) The cleanliness of our air — or lack thereof — affects our weather, said Wei Gong, another study co-author and a physicist at Wuhan University in China . “Aerosols in clouds have a significant effect,” he said.

Next, Dr. Pan and his colleagues found that larger particles, particularly chunks of sea salt produced by windswept spray, suppress lightning. These aerosols — at least 10 times larger than air pollution — also attract a lot of water, said Dr. Rose field. “They absorb a lot of water vapor and form relatively large droplets,” he says.

But these powerful drops are more likely to rain from clouds. Because this process removes a key ingredient in how lightning forms, sea spray is essentially a lightning killer, the team suggests.

dr Pan and his collaborators showed that the spray’s effects were significant: storms exposed to high concentrations of sea salt aerosols produced up to 90 percent fewer lightning bolts than storms with very low concentrations of sea salt aerosols. This significant drop came as a surprise. “I didn’t expect it to be that big,” said Dr. Rose field.

The researchers concluded that thunderstorms are therefore rarer over the ocean than over land for two reasons. The fine airborne particles that fuel lightning are more common over land, near more pollution sources. And larger sea salt aerosols — which suppress lightning — occur naturally near or over open water. That’s a double whammy, said Yannian Zhu, an atmospheric physicist at Nanjing University in China and a co-author of the study. “Different aerosols have significantly different effects,” he said.

These results offer a tantalizing explanation for a well-observed phenomenon, said Dr. Romps from the University of California, Berkeley. However, clouds don’t give up their secrets anytime soon, he said. This work is just an exploration of what is sure to be a long line of research. “That won’t be the last word,” he said.

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