YOU won’t find this species in your average cloud-spotter’s guide. These fluffy strings might look like icy contrails left by aircraft, but they are actually created by ships plying the ocean.
Clouds are collections of tiny water droplets, which need a speck of something solid to condense around. That can be anything from bacteria to salt particles.
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Ships throw out sooty particles as they burn fuel. These polluting particles seem to seed clouds with a greater density of smaller droplets than normal, so the ships leave highly reflective tracks.
Like contrails, the ship tracks betray the paths of the vessels that created them. Here, off the coast of Portugal and Spain, many ships are forging north towards ports in Europe. Their tracks have merged into a band of cloud to the right of the photo. Other individual east-west trails, some of which stretch hundreds of kilometres, belong to ships crossing the Atlantic.
They may be beautiful, but these clouds cast a dirty shadow. Cargo ships emit little carbon dioxide compared with other modes of transport. But they use bunker fuel, a cheap and sludgy oil that can emit about 3500 times as much sulphur dioxide as diesel does. Sulphur dioxide contributes to acid rain and can cause respiratory problems. That is one reason why bunker fuel is banned in many coastal shipping routes, including the English Channel.
Photograph
NASA/Jeff Schmaltz, LANCE/EOSDIS Rapid Response
This article appeared in print under the headline “Sky liners”
