What is the vapor trail about with jet aircraft?
Most of the above answers are marginally correct as to what causes these. It is water vapor or water vapor converted to ice crystals. Contrails are only created from the engines (usually turbine but yes WWII bombers flew so high their engines did this as well). It happens. The process happens when the engine exhaust increases the water content in the air past the saturation point (pretty easy at high altitude). This turns to ice crystals at such low temperatures that is why the really high ones tend to stick around a long time and sometimes catalyze a cloud. Why some do and some don’t? A bunch of reasons: – Different altitude (hard to judge when you are on the ground).
