The “smoke” trails that appear behind some aircraft are actually known as contrails and are in fact water vapor. The warm moist air that comes from the jet mixes with the cold air in the sky. It creates a condensation trail or contrail for the same reason you can sometimes see your breath.
Some people say you can predict the weather with contrails. If you see one and it dissipates quickly, there is low humidity and you can expect fair weather. But if it is thick and hangs around for a long time, then there is high humidity and rain may be in the forecast.
