system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes the trail behind jets at high altitude?

Physics teacher today said it was to do with the pressure difference around the wing creating a disturbance which creates a 'cloud' behind the plane. I always thought it was water vapour (or something similar) coming from the engines and freezing since it experiences such low temperatures so high up.
gpt
It is water vapor and ice. They are produced from the hot engine exhaust in the cold atmosphere. Water vapor from the engine exhaust mixed with unburnt particulate in the jet fuel gives the surrounding moist air something to latch onto and ice crystals form. Depending on the hight of the aircraft, they can last seconds to hours.

If you have seen a running car on a brisk morning, that is a similar effect. The car is too close to the relatively warmer ground that trails do not last for more than a second.