I take it you mean the vapour trails from aircraft flying at high altitude. The hot, humid exhaust from jet engines mixes with the atmosphere, which at high altitude is of much lower vapour pressure and temperature than the exhaust gas. The water vapor contained in the jet exhaust condenses and may freeze, and this mixing process forms a cloud very similar to the one your hot breath makes on a cold day.

The appearance of a vapour trail is all dependant on exterior air humidity. After some time, this humidity is absorbed and contrails disappear. High humidity = Vapour trails