That giant cargo ship isn’t just belching plumes of exhaust; it may be seeding clouds primed for lighting strikes. In a study published today in Geophysical Research Letters, researchers calculated average yearly rates of lightning in the northeastern Indian Ocean and the South China Sea for the years 2005 through 2016. They found that two major shipping lanes (one between Sri Lanka and the northern tip of Sumatra, and another stretching northeastward from Singapore past southern Vietnam) experienced nearly twice the number of lightning strikes as similar strips of ocean a few hundred kilometers away. Because the areas of enhanced lightning are much wider than the shipping routes themselves, the higher rates of lightning probably aren’t due to bolts striking ships directly. The increase in lightning rates also doesn’t seem to be triggered by changes in weather. Instead, the team proposes, soot and other particles in the ships’ exhaust create large numbers of cloud droplets that are, on average, smaller than those forming around natural dust particles in the air elsewhere over the ocean. Because those smaller droplets tend to rise higher into the atmosphere, they ultimately create a larger number of ice particles—which, in turn, rub against each other to generate lightning. Although the sooty exhaust tended to boost lightning rates, other data suggest that the storms didn’t produce more rainfall than those over areas nearby, the researchers say.