# Airplane trails

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?

• 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. – zeta-band Feb 26 '18 at 21:35

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.