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Published on Feb 4, 2008
From the book "A color guide to clouds", published in 1963:
"Contrails persist when they are formed at a sufficiently low temperature for them to become frozen before they evaporate. Then because of different winds at different levels, they may be spread over the sky to form a layer of artificial cirrus. Sometimes the air is sufficiently dry for the ice cloud to evaporate in ten minutes or so; but often it may last for several hours"