Contails

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http://en.wikipedia.org/wiki/Contrail

Contrails (short for "condensation trails") or vapor trails are long thin artificial clouds that sometimes form behind aircraft. Their formation is most often triggered by the water vapour in the exhaust of aircraft engines, but can also be triggered by the changes in air pressure in wingtip vortices or in the air over the entire wing surface.[1] Like all clouds, contrails are made of water, in the form of a suspension of billions of liquid droplets or ice crystals.

Depending on the temperature and humidity at the altitude the contrail forms, they may be visible for only a few seconds or minutes. Persistent contrails are thought to have a significant effect on global climate.