I know I learned this in school but I cannot seem to recall what is happening in the "upper level of the atmosphere" when a white "trail" shows behind airplanes. Is it colder air? Kim

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MIKE MOSS SAYS:    Kim,    The white trails you are thinking of are called contrails, short for "condensation trails." Contrails form when water vapor in aircraft engine exhaust condenses into clouds composed of ice crystals upon mixing with outside air in the wake of the plane. They are more likely to develop and are more persistent when upper level air temperatures are especially cold and relative humidity there is high. Ambient wind speeds and turbulence levels impact the shape and duration of contrails as well. Whether they form or not is also influenced by the atmospheric pressure at flight level and the temperature and moisture characteristics of the jet engine exhaust itself.

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