(This won't convince the nutters, but I find aviation and meteorology neat)
Contrail. Short for Condensation Trail.
There are two kinds condensation trails I'm aware of,
wingtip vortices and
regular exhaust trails.
What happens when you take a icy cold drink, and leave it outside on a hot and humid day? It develops condensation (as if by
magic). The icy beverage drops the surrounding air temperature to the dew point (point of saturation, where water turns from a gas to a liquid and becomes visible).

Contrails are kind-of similar in that they too produce condensation. In Wing Vortices, air rolls off of the wings due to their shape, lift, etc and enter into a fast swirling motion like so:

In the center of the swirl (the part that looks like the eye) air pressure drops. When pressure drops, temperature also drops (sometimes beyond the dew point). Just like a cold bottle of beer dropping the air temperature and producing condensation, the cold core of the vortices can also produce visible condensation. Like so:

Since clean air is invisible to the eye, we can't really see the vortices without using some kind of way to dirty up the air (smoke, fog, clouds etc). You can youtube demonstrations of this. But we can occasionally see the condensation trails they produce.
The other kind of contrail is through engine exhaust. The two main products expelled by the engine are CO2 and water vapor.
Anything above 24,000 feet will always freeze... and voila!
This post was edited by Black487 on Mar 26 2013 04:49pm