Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust. Cloud formation by a mixing process is similar to the cloud you see when you exhale and "see your breath".

What are contrails caused by?

A contrail will form if, as exhaust gases cool and mix with surrounding air, the humidity becomes high enough (or, equivalently, the air temperature becomes low enough) for liquid water condensation to occur. ... If sufficient humidity occurs in the exhaust plume, water con- denses on particles to form liquid droplets.

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.