Near Miss over Paris?

A crystal clear (and frigid) morning in Paris. Virtually no jet trails around, except for this pair low in the east. Viewed large, the outlines of the planes are clearly visible (even the foreshortened wings of the one on the right).

The shot was at 8:56 (official sunrise was 8:18--the sun takes a long time to get above the high buildings near Chinatown, so it hadn't quite reached us). Quite a contrast with this large group of afternoon trails earlier this month. Here's a more recent related blip.

I'd be interested to hear from someone under what conditions jet trails persist, and when they dissipate very quickly, as these two clearly did. The weather was certainly warmer for the earlier blip--could that have been a factor?