The British did contrail studies in WWII, but for reasons other than weather. Photo-Recon Spitfires (the SR-71 of WWII) would expose themselves if they started their recon flight over occupied Europe while in contrail inducing atmospheric conditions (yes piston engines can and do produce contrails).
If the contrail/weather theory has any validity than the trails of hundreds of B-17's and B-24's, day after day for 3 years should have effected Europe's weather during the war and disaffected it after.
I agree with the other poster, I'd guess that there was a significant change to the regular usage of that dreaded internal combustion engine during that time frame as well. I seem to recall lots of stories about people not going shopping , or to other types of entertainment. Seems like a lot fewer cars on the road to me. ?????