What is a contrail (jet)?

A jet plane leaves a contrail in flight for the same reason that you can sometimes see your breathing. Hot, wet exhaust from jet engines mixes with an atmosphere that, at high altitude, has a much lower vapor pressure and temperature than the exhaust gases. The water vapor contained in the jet exhaust condenses and can freeze, and this mixing process forms a cloud much like your hot exhalation on a cold day. Jet engine exhaust contains carbon dioxide, sulfur and nitrogen oxides, unburned fuel, soot and metal particles, and water vapor. The soot provides areas of condensation for water vapor. Any particles in the air provide additional areas.