Condensation trails
Maybe a pilots' site isn't the best to ask this question, since you, of all folks, never see your own! But here goes anyway.
Speaking from a land-lubber's perspective.... Although I do fly (a paramotor).
Some days there are no contrails to be seen, some days they last for a short time then disappear, and some days there are multitudes of them in the sky.
What makes the difference? Isn't always pretty cold up there?
Next, a couple of weeks ago I watched 2 planes making very well defined twin trails. As I watched a fuzzy much wider one could be seen, apparently above the two sharp ones. It looked like an old trail, but it wasn't there in front of either plane. Could it be the apu?