It's common to see airplane "contrails," or condensation trails, stretched across the sky. They look a lot like the elongated clouds in the photo above — except those aren't from airplanes. (They aren't roll clouds, either, despite their similarity to this photo.)
According to NASA, the clouds pictured above were created by ships off the U.S. West Coast. The photo was taken by NASA's Aqua satellite on Feb. 21, revealing a rarely seen maritime twist on familiar airplane contrails.
Clouds normally form when water vapor condenses around a tiny particle, like dust or salt. Ships can spur this process by burning fossil fuels and emitting small aerosol particles, which serve as nuclei for the formation of new clouds. Because the ships are moving quickly across the ocean, these cloud nuclei are left behind like a trail of bread crumbs, providing a "Family Circle"-style track of a ship's travels.
Here's a more detailed explanation from NASA:
"In order for such clouds to form, three things must occur. First, the ship must emit small particles in the exhaust, which provide the cloud condensation nuclei. Second, there must be very humid air in the ship's path, and third, the air surrounding the vessel must be non-turbulent. All three conditions were met on the day this image was captured."
NASA has posted photos of "ship tracks" before, including one over the Atlantic Ocean in 2005 and one over the northern Pacific in 2002. The agency says "the exhaust released by ships is not a significant source of pollution," although a 2011 study did suggest airplane contrails may compound the effects of global warming.
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