Well in this case as I explain planes are up there on their way to close airport or on their route to other places, if air is so still and there is inversion you can actually see the condensation or vapor trails. In this case they are called contrails nothing to do with the chemtrails that is considered in most cased not true to the facts ;-) if you want to be at ease and relieved about that :) Hot humid air from the engine exhaust mixes with the colder surrounding air, and causes water vapor to condense into water droplets and ice that form visible clouds, the time they stay depend on weather conditions. If you look up to the sky enough and happen to see a regular plane you will notice in certain weather these are likely to be left behind. Sadly it's not my favourite in pictures but a human "mark" into the sky, but except for the contaminants of the engine combustion :) (bad enough) nothing to worry about them :)