The Tracks of Ships Are Written in the Sky

Posted: May 16, 2013

By: Phil Plait [Slate]

Here’s something I didn’t know happened: Under the right conditions, the exhaust from ships plying the ocean can form clouds, leaving tracks criss-crossing the sky.

This image, taken by NASA’s Earth-observing Terra satellite on Apr. 20, 2013, shows some of these long thin clouds (called ship tracks) in the Aleutian Islands off the coast of Alaska. Actually, there’s quite a bit going on here, and the ship tracks are just one part.

The tracks themselves can be seen as the mostly linear clouds all over the bottom of the picture. What happens is that the ships emit aerosols—tiny particles—in their exhaust. Water vapor in the air condenses around the particles, forming tiny spheres of water: cloud droplets. As the ship moves, these trace its path like airplane contrails, and over time the ship tracks can have their shape bent by winds.

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