Why does a white trail sometimes appear behind a plane flying at high altitude?

A flying plane leaves behind the combustion products of aviation fuel, and water vapor is also present in the composition of these products.

At high altitudes, where the air is highly discharged, the temperature is low. The plume of water vapor left by the aircraft freezes, turning into ice crystals.

With a sufficiently high concentration, such a plume will have a white color (by analogy with snow, where each snowflake individually is an icy colorless crystal, but in its mass all snowflakes are perceived by our vision as something white).