Planes leave a trail, called contrails (short for condensation trails), which are line-shaped clouds formed by the water vapor produced from the combustion of fuel in aircraft engines. At high altitudes, where the air is very cold (around -55°C or lower), this water vapor condenses and freezes into tiny ice crystals, creating visible white trails behind the aircraft. The length and persistence of these trails depend on the humidity and temperature of the surrounding air. In very dry air, contrails quickly dissipate, while in more humid air, they can spread out and last longer, sometimes resembling natural cirrus clouds.
These contrails form because the hot exhaust gases from the engines contain water vapor and particles, which act as nuclei around which ice crystals form when the ambient conditions are cold enough. Contrails generally appear at cruising altitudes several kilometers above the Earth, where the temperatures are low enough to allow this condensation and freezing process.
In summary, the visible trails planes leave in the sky are basically clouds created from the aircraft's exhaust in cold, humid conditions at high altitudes.
