What causes the "white smoke" trailing high flying jet planes ?

Modern turbofan (jet) engines actually mix with fuel only a small portion of the air they suck in. The vast majority (90% or so) of air is "bypass air" that is compressed and then used for additional thrust. This air expands as it leaves the engine.

All engines that burn petroleum-based fuels (jets, piston engines, etc.) produce steam and CO2 as their exhaust. The CO2 is invisible on exiting the engine and stays that way, but in cold air, the steam condenses into liquid water or ice, forming a white mist that leaves a trail behind the aircraft. This trail is called a condensation trail, or contrail.

Contrails may or may not be visible, depending on atmospheric conditions. They are common above 26,000 feet, but uncommon below. Again, depending on atmospheric conditions, they may be completely absent, or they may disappear after a few minutes, or they may grow into ordinary cirrus clouds.

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