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Originally Posted by C_Duff
Are you serious? Do you actually believe this BS?
Actually, you'll see more contrails when there is more upper atmosphere mositure. When the moist air is heated in the engine, then cools as it exits, the moisture is what freezes into ice crystals and becomes the contrail
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Elaborating... I've heard that most of the moisture in a visible contrail is actually already in the atmosphere before the jet engine passes through. What happens is that the engines produce yet more water vapor as byproduct of combustion and if the local atmosphere at the flight level is already fairly saturated, the jet exhaust often adds just enough extra moisture to raise the dew point above the resulting atmospheric temperature as the mixture of local atmosphere and exhaust gasses mix and then cool.
Eventually, the moisture disperses into the surrounding, somewhat drier air just enough to no longer drop below the dew point and the contrail basically dries back up. How long this takes might be bit of a function of the aerodynamics of the plane and engine, which determines how well the exhaust gasses mixed into the local atmosphere. Mainly, however, it's a function of how close the atmosphere was to the dew point before the jet passed by, as well as what the prevailing winds aloft were after the plane passed by that determine how long the elevated moisture trail takes to dissipate, dropping the dew point again. Sometimes the contrails last a minute or so after the plane passes, (indicating a medium amount of pre-existing water vapor), sometimes there isn't any contrail at all, (indicating a "dry" atmosphere at that flight level), and sometimes they can last an hour or more (indicating a more closely "saturated" atmosphere. If the air is already completely saturated, you already have visible moisture, aka a cloud.
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Again, it depends mainly on how close the atmosphere already is to the dew point before the plane passes through the area (and at the altitudes typical of contrails, "dew" usually means ice crystals, which I suppose might last even a bit longer afterwards too.)