Jets leave white trails, or contrails, in their wakes for the same reason you can sometimes see your breath. The hot, humid exhaust from jet engines mixes with the atmosphere, which at high altitude is of much lower vapor pressure and temperature than the exhaust gas.

Do all planes leave contrails?

Contrails form when jet exhaust emits water vapor that condenses and freezes. Contrails don't form for every airplane. The atmosphere where the plane is flying needs to have low vapor pressure and low temperature. There are three types of contrails.

What causes jet contrails?

Planes create their mesmerizing contrails as they soar high in the thin, cold air. Water vapor quickly condenses around soot from the plane's exhaust and freezes to form cirrus clouds, which can last for minutes or hours.