system
Given the following post, respond as if you're commenting on Reddit:
human
What is the difference between the streaks left behind from plane/jet engines that disappear quickly, and the ones that stay there for hours?
gpt
Depends on the moisture content of the air in that layer.  If there is a lot of moisture, a contrail will form, that is basically an artificially created cloud.  That cloud can then be dispersed by upper level winds, so the length of time it remains varies.  When you see a jet overhead at high altitude and the contrail disappears almost as fast as it forms, that means that it is pretty dry in that layer of air, and there isn't enough moisture to form a strong enough cloud to maintain itself, though the disturbance in the air can condense it for a short time.  It is not the exhaust of the engines that forms the contrail, it is the disturbance of the air causing moisture to condense.  That is why propeller-driven aircraft can form a contrail at high altitude as well, and a contrail can form from the wingtip vortex created by the wing moving through the air in addition to the disturbance from the engines.