It's called getting from point A to point B. Have you ever timed the arrivals and departures from a major airport? They come and go, sometimes minutes apart. And they are aligned to a common track into the airport. Plus, as another anon pointed out, the weather conditions (cirrus clouds) would allow contrails to persist for hours on end, plenty of time for other airplanes to pass by. All the flights from Chicago to Los Angeles, for example, would follow the same Great Circle path. Why would they go out of their way?
To be honest, I have real doubt that a temperature inversion would extend to such an altitude. In my experience, they have been low altitude phenomena.
Because they are established flight paths.
What is the purpose of flying those paths in such a tight time interval?
It's called getting from point A to point B. Have you ever timed the arrivals and departures from a major airport? They come and go, sometimes minutes apart. And they are aligned to a common track into the airport. Plus, as another anon pointed out, the weather conditions (cirrus clouds) would allow contrails to persist for hours on end, plenty of time for other airplanes to pass by. All the flights from Chicago to Los Angeles, for example, would follow the same Great Circle path. Why would they go out of their way?
That makes sense. I suppose a temperature inversion/stagnant air would also allow the lines to stay all day.
To be honest, I have real doubt that a temperature inversion would extend to such an altitude. In my experience, they have been low altitude phenomena.