"Rectangle cloud" made me LOL. That's actually really sad, but I needed the giggle. 😂
The bit that my book said was that if it's a water vapor contrail, it can last for up to a half hour. So if you see it last longer than a half an hour, I would be putting my questioning cap on! But, it didn't go into too much detail; I presume they mean it stays a line for up to a half hour. I don't know if that "smear" counts in the time measurement, or not.
Either way, it can only be useful to have more information, whatever it reveals. We need to do a steemit fundraiser to get you a time lapse camera!