Doesn't the presence of con trails indicate moist air rather than dry air? On these 'noisy days' the con trails are quite visible if not quite as long or long lived as on other days.
It would seem that today there is enough moisture to allow con trail formation, but not enough to sustain them for more than several seconds.
Last edited by Shaggy Sheep Driver; 28th Dec 2017 at 15:29.