This was the conclusion reached by Joel Thornton of the University of Washington, Seattle, and his colleagues in a paper just published in Geophysical Research Letters.
Dr Thornton and his team considered 1.5 billion strikes recorded in this part of the world by the World Wide Lightning Location Network (an international collaboration led by Dr Thornton?s colleague, Robert Holzworth) between 2005 and 2016, reported London?s Economist.
As the map shows, the strikes that occurred over the ocean were concentrated in places most plied by ships. In particular, the shipping lane that passes from the south of Sri Lanka to the northern entrance of the Straits of Malacca, and thence down the straits to Singapore, positively glows with lightning. So do the lanes from Singapore and the western part of Malaysia that head northeast across the South China Sea.
Neither changes in vertical wind shear nor differences in horizontal air movements seem likely to be causing this concentration of thunderstorms, for other measurements suggest that these weather-modifying phenomena are the same inside shipping lanes as they are in neighbouring parts of the atmosphere immediately outside those lanes.
Nor does it seem plausible that the ships themselves are responsible for attracting all the extra strikes involved. Though the area of the lanes is small compared with the whole ocean, it is vast compared with the area actually occupied by vessels. Most of the extra bolts are hitting the sea rather than craft sailing across it.
The most likely explanation is particulate pollution emitted by the ships using the shipping lanes. Marine diesel burned offshore is generally high in sulphur, and its combustion produces soluble oxides of that element which act as nuclei for the condensation of cloud-forming droplets.
Typical marine clouds in unpolluted areas are composed of large droplets and do not rise to high altitude, but Dr Thornton and his team reckon that smaller droplets, of the sort that condense around oxides of sulphur, might more easily be carried upward by convection?forming, as they rose, into towering storm clouds that would act as nurseries of lightning bolts.