PC,
I have to say, for argument's sake only, that the videos of trails coming from the planes only shows the immediate space directly behind these planes, and that it only shows that trails of any kind are created by these high-altitude planes. It doesn't really show that they persist or spread. Playing the devil's advocate, if you don't mind, causes me to say that even the pictures that show a trail from one side of the picture to the other does little to indicate that they were there for longer than a couple of minutes (although a couple of them certainly look like they would persist - again, an assumption). I witness trails that last longer than the short, quickly dissipating trails, but do not linger for hours or cover the sky. So, there seems to be a great degree of variation, even in just what I witness, personally.
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