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When Joel Thornton at the University of Washington in Seattle and his colleagues looked at records of lightning strikes between 2005 and 2016 from the World Wide Lightning Location Network, they noticed there were significantly more strikes in certain regions of the east Indian Ocean and the South China Sea, compared with the surrounding areas. Unusually, they occurred along two straight lines in the open ocean, which coincided with two of the busiest shipping lanes in the world. Along these paths there were twice as many lightning strikes as in nearby areas.

“We were quite sure the ships had to be involved,” says Thornton. But they still had to eliminate other factors that influence storm intensity, such as wind speeds and temperatures.

Once these had been ruled out, the team concluded that aerosols from the ships’ engine exhausts were the culprit. Aerosol particles act as seeds, around which water vapour condenses into cloud droplets. In clean air there aren’t many seeds, so the cloud drops quickly grow and fall as rain.

But when there are a lot of seeds, like over busy shipping routes, a greater number of small cloud drops form. Since these are light, they rise up high into the atmosphere and freeze, creating clouds rich in ice.

It is this that leads to more intense thunderstorms: lightning only occurs if clouds are electrically charged, and this only happens if there are lots of ice crystals.

A key giveaway that aerosols were behind the effect was that the lightning was most pronounced at times of the year when powerful atmospheric convection currents form that can carry the aerosol particles high into the sky (Geophysical Research Letters, doi.org/cc7b).

Although lightning activity is higher over the shipping lanes, the amount of rainfall is no different to nearby regions.

More: https://www.newscientist.com/article/mg23531442-300-lightning-storms-triggered-by-exhaust-from-cargo-ships/

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