A saying by ancient Chinese philosopher Lao Tzu that “if lightning is the anger of the gods, then the gods are concerned mostly about trees” needs to be revisited after research found that salty sea spray stops lightning from happening during thunderstorms.
This might explain why lightning happens much more over land than it does over the sea, exactly as implied by the old master.
The research, published in Nature Communications, suggests that coarse aerosols from sea spray reduce the frequency of lightning by 90% in tropical storms. Aerosols are tiny particles of liquid or solid, so small they float easily in air.
The international team of researchers examined data from a large region of the tropics: between 50°W and 50°E, 20°N and 20°S, across the African continent and both adjacent oceans.
They looked at lightning strikes from the World Wide Lightning Location Network, alongside a range of other meteorological data, over a five-year period from 2013 to 2017. They also examined aerosol concentrations, using NASA’s MERRA-2 dataset.
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While fine aerosols (smaller than a micrometre) increased lightning frequency on both land and sea, coarse aerosols from sea spray had the opposite effect. Even when all the other weather and cloud conditions were similar, higher amounts of sea spray aerosols reduced the frequency of lightning strikes.
In contrast, coarse aerosols that occur (more rarely) over land, such as those cause by desert dust, had a slight encouraging effect on lightning.
In their paper, the researchers suggest that the reason coarse sea spray reduces lightning is that the aerosols encourage fewer and bigger water droplets to form, which increases the amount of warm rain in a cloud. This means there are fewer ice crystals, which help clouds to electrify.
“The probable mechanism of the coarse sea salt aerosols is by enhancing drop coalescence and replacing mixed-phase precipitation with warm rain, thus “killing” cloud electrification,” write the researchers.
They add that tracking marine aerosols more closely would allow meteorologists and climate scientists to make more accurate weather and climate predictions.