They’re not directly correlated, but generally, when you see lengthy contrails behind an aircraft, it’s probably cruising at altitude. Again, not directly relat

They’re not directly correlated, but generally, when you see lengthy contrails behind an aircraft, it’s probably cruising at altitude. Again, not directly related – it’s an atmospheric phenomenon[0] – but visible contrails are a safe indicator an aircraft is at or near cruise and are quite common. (Their absence, on the other hand, is not necessarily a counterindication.) And yes, an aircraft will pretty much always be visible to the naked eye, even up to 45-50k where some business jets live now. You may have to squint, but your eyes are pretty good at finding fast motion against a static background. [0]: https://en.m.wikipedia.org/wiki/Contrail https://en.m.wikipedia.org/wiki/Contrail