Why do jet airplanes leave white trails in the sky?

Jets leave white trails, called contrails, in their paths for the same reason you can sometimes see your breath on a cold winter morning. The hot, humid exhaust from jet engines mixes with the atmosphere, which at a high altitude is of much lower vapor 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. Depending how high the plane is flying, and the temperature and humidity of the atmosphere, contrails may be thick or thin, long or short. The different types of jet contrails can be used to predict the weather. For example, a thin, short-lived contrail indicates low-humidity air at high altitude, a sign of fair weather, whereas a thick, long-lasting contrail reveals humid air at high altitudes and can be an early indicator of a storm.