This is interesting, if just because it is a decent demonstration of something that seems beyond a "contrail". Contrails will normally form from either water vapor from the engines, or low pressure edges from wing tips and appendages. This seems to indicate a cutoff of material being dispensed vice a sudden change in atmosphere. If you look at old bomber film, or even 50's/60's film, it was always above 20,000 feet, and because of cold, vapor laden air.
SOURCE URL: https://www.youtube.com/watch?v=q1EU7IaMM_k