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Some airplanes leave a trail through some regions of sky. This post explains why it is so, but it seems not enough.

The trail is said to be made mostly of water which, on a specific altitude, depositates (goes from gas to solid). So the explanation can be made in terms of pressure and temperature needed to form ice crystals. I think pressure also explains the length of the trail. But it can be read that, in order to form crystals, the air needs to be wet. There are two issues I cant understand:

Why air needs to be wet? Why there is so much water in the airplane gases?

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  • $\begingroup$ Jet fuel is composed of hydro-carbons of length 8 to 16. Burn it in air and you are going to get CO2 and H2O as the principal products. $\endgroup$ – zeta-band Feb 26 '18 at 21:35
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By "wet", it is meant that the air contains significant amounts of dissolved water vapor. This condition is important here for several reasons, as follows. First, some background:

The exhaust plume from a gas turbine contains lots of hot water vapor because the two main products of combustion of jet fuel are $\rm CO_2$ and $\rm H_2O$. At the high altitudes at which jets cruise, the ambient temperature is low enough to freeze water vapor into ice crystals; But if the amount of water vapor already present in the ambient air is very low, the vapor from the engines upon freezing will quickly sublimate into the "dry" air and disappear, or mix quickly with the cold air and dissolve into it without first forming ice. In these cases, a visible ice trail from the engine either never forms or very rapidly disappears.

If, on the other hand, the ambient air contains lots of vapor, the sublimation process is greatly slowed down, and the ice crystal formation process is enhanced. These effects then allow for a dense plume of ice crystals to form in the engine's exhaust and persist for many minutes or even hours after creation!!

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  • $\begingroup$ Thanks, but I don't understand some things. Why exhaust plume contains water as a product? I mean, water a strange product, isnt it? What is the fuel made of? And why does water sublimates if temperature is low enought to freeze? $\endgroup$ – santimirandarp Feb 26 '18 at 18:05
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    $\begingroup$ @santimirandarp, the fuel is kerosene (a.k.a., "paraffin")---a mixture composed mostly of unsaturated hydrocarbons. That is to say, chemicals consisting entirely of carbon atoms (C) and hydrogen atoms (H). Those combine with oxygen (O) from the air when the fuel is burned, and the result is what niels nielsen said: CO2 (a.k.a., carbon dioxide) and H2O (a.k.a., water). $\endgroup$ – Solomon Slow Feb 26 '18 at 18:14
  • $\begingroup$ The cold air at the altitudes where contrails form never contains a lot of water vapor. (The limit is called vapor pressure, and it is an exponential function of temperature.) However, the air up there is usually saturated, i.e., 100% relative humidity. $\endgroup$ – Bert Barrois Feb 26 '18 at 21:36
  • $\begingroup$ @bertbarrios, yes indeed, but I left out the relative humidity argument because I did not assume the OP knew about that. $\endgroup$ – niels nielsen Feb 26 '18 at 22:29

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