One of nature’s most intense spectacles can be softened with the humble sea salt, the same that is present at dinner tables.
Researchers have recently discovered that the frequency of lightning and lightning decreases by up to 90% in the presence of salt spray marine water. It makes sense, they propose, based on how electric fields build up inside clouds. These new results, presented this month in the journal Nature Communications, help explain why lightning and lightning storms are much less frequent in the ocean than on land.
Sailors have a lot to worry about, from billows to floating garbage. But at least being struck by lightning doesn’t usually figure on the list of the most common hazards they face: compared to the lightning and lightning recorded over continents, only a tenth of them occur at sea. The exact reason for this has been an enigma for a long time. And that’s a “glaring gap” in our understanding, said physicist David Romps of the University of California at Berkeley, who was not involved in the research.
To try to solve the riddle, Zengxin Pan, an atmospheric physicist at the Hebrew University of Jerusalem, and his colleagues mined data from more than 75,000 recent lightning storms. The researchers combined observations made by the World Wide Lightning Location Network, a global repository of radiofrequency lightning observations, with satellite-based measurements of cloud properties, precipitation and tiny airborne aerosol particles.
The aim was to identify how convective clouds — where lightning is born — evolved over time in different atmospheric conditions, said Daniel Rosenfeld, co-author of the study, also an atmospheric physicist at the Hebrew University of Jerusalem.
First, the researchers demonstrated that aerosols smaller than a millionth of an inch — such as the dust and soot particles associated with air pollution — tended to increase the frequency of lightning for a given amount of rain. That was to be expected, Rosenfeld said. Fine aerosols act as tiny airstrips for water contained in clouds. The liquid tends to stick to them, forming droplets, but those droplets are small enough that they tend to stay inside a cloud rather than falling to the ground as raindrops, Rosenfeld added.
And because the presence of water in clouds creates intense electric fields that lead to lightning, dirty air leads to more lightning — this effect has previously been demonstrated on shipping lanes, traveled by ships that spew pollution. Air purity (or impurity) affects our weather conditions, said Wei Gong, another study co-author and physicist at Wuhan University in China. “Aerosols in the clouds have a significant effect,” he said.
Next, Pan and his colleagues demonstrated that larger particles, specifically the sea salt particles produced by wind-blown marine spray, suppress the rays. These aerosols, at least ten times larger than air pollution, also attract water, lots of water, Rosenfeld said. “They absorb a lot of water vapor and form relatively large droplets,” he explained.
But these heavy drops are more likely to fall from clouds as rain. Because this process removes a key ingredient in lightning formation, marine spray is essentially a lightning and lightning killer, the scientists suggest.
Pan and his collaborators have demonstrated that the effects of sea spray are substantial: storms exposed to high levels of sea salt aerosols produce up to 90% less lightning and lightning than storms with very low levels of sea salt aerosols. This sharp reduction came as a surprise. “I didn’t expect it to be this big,” Rosenfeld commented.
Thus, the scientists concluded, lightning storms are less common over the ocean than over land for two reasons. The fine airborne particles that promote lightning are more prevalent over land, close to more sources of pollution. And the larger aerosols of sea salt, which suppress lightning, are found naturally over or near the open sea. It’s a double effect, said atmospheric physicist Yannian Zhu of Nanjing University in China and a co-author of the study. “Different aerosols have significantly different impacts,” he said.
These results offer an interesting explanation of a widely observed phenomenon, said Romps of the University of California at Berkeley. But clouds don’t reveal their secrets easily, he cautioned. This work is just an investigation into what will surely be a long line of research. “It won’t be the last word,” he said.
