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?