Flight Lines - Why Contrails Hang Around

From Air & Space Magazine:

In John Ford's last western, Cheyenne Autumn, long threads of white cross the sky over army tents in Monument Valley, along the Utah-Arizona border. In the 1993 film Gettysburg, a brilliant white sliver hovers in the clear sky above the head of a Union officer. In Zulu, a dramatized account of the 1879 assault by thousands of Zulu tribesmen on a small British garrison in Natal, South Africa, whitish bands sometimes hang in the sky beyond the hills. Nitpickers who enjoy finding movie bloopers point out that these man-made cirrus clouds are condensation trails, or contrails, which didn't exist before the 20th century. They are created by airplanes flying at high altitudes, where the air is below -38 degrees Fahrenheit. Exhaust from airplane engines contains water vapor as well as other gases and particles of soot and metal. When the exhaust is expelled into and mixes with the cold air, the water vapor condenses into droplets, which instantly freeze into tiny ice crystals. What you see from the ground is a dense white stream of ice crystals behind an airplane.

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