system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some airplanes leave a visible contrail and others don’t?
gpt
It does not depend so much on the airplane as the humidity, pressure and temperature of the air in flies in. Contrails is an atmospheric effect that is caused by the increased humidity from the engine exhaust raising the humidity in the air high enough to form clouds. There is a few other effects such as sot creating nucleation points for the ice crystals and turbulence creating low pressure spots for crystals to form but these are of minor significance. Contrails will be formed whenever an airplane moves through air that is close to forming clouds already. The humidity from the exhaust is just the final straw that makes the clouds form behind the airplane. If you see two airplanes in the same sky where one forms a contrail and the other do not then it is likely because they are passing through different layers of the sky with different conditions.