I noticed that yesterday in West Yorkshire while removing external Chritmas lights, that the high level A/C heading either out of Europe for the Atlantic and the large ex LHR 747 heading up country for American Wext Coast and SW were noticably louder. Conditions were cold at 1c but no cloud at all with straight vapour trails visable behind each A/C for about 20miles. The A/C sound also seemed to lag further behind where the A/C was by more than normal in my observations.
Regards
Mr Mac