Contrails..... a "shorthand" for
Condensation Trails. The Hot Gases exiting a Jet Engine leave a "Vapour Trail" across the sky. It is merely the hot air [exhaust] meeting the cold air [atmosphere], and being visible. At different altitudes, the Con/Vapour trails are NOT visible due to atmospheric conditions...[temperature/pressure gradient..etc]. Converely, sometimes, due to the same "atmospheric conditions".....Con/vapour trails seem to last for eveeer ........
hope this helps
watp,iktch
thanx hobie