Two heavies crossing above Cleveland last March. Photo: Chuck Slusarczyk Jr. – OPShots.net
Contrails are generally classified into two types. Exhaust contrails and aerodynamic contrails.
Exhaust contrails are formed by the mixing of the hot humid exhaust of the engines with cold humid surrounding air, creating long streamers of clouds. If the conditions are right then these can persist and spread. These are the most common type of contrail observed.
Aerodynamic contrails are formed by the temporary reduction in pressure of the air moving over the surface of the plane, or in the center of a wake vortex. Reducing the pressure of the air means it can hold less water, so condensation occurs.
via Contrail Science.
