Contrails
 

Reason is simple: contrails can be seen 100 Nm away, even made by a very small fighter plane. Mil pilots don't want to be seen/killed. During pre mission climbout, the first element of the flight who'd see trails would call it out and call the moment they would stop as well. This defines an altitude block you don't want to work in if you don't want to be spotted visually. A prediction can be made by the weather forecaster but is generally inaccurate because not every engine will trail at the same altitude/time of day, and, closing in to a frontal system, the humidity in the air varies rapidly so the altitude at which the trails will form will vary accordingly.

If you want to organize a film event, the best method is to fly in formation and call out the first trail you see, remember the altitude, descent back to 1000' lower than that level, resume climb and start filming.:D