In response to a local newsgroup's Facebook post about contrails being chemtrails, that is, some sort of government plot to...well, I actually don't know what it's about, but apparently it has something to do with chemicals in the sky...anyway, I replied:
During WWII, before the Mustang began escorting bombers all the way to the target and back, Luftwaffe fighter pilots found the bombers easily by following their contrails. They referred to them as die dicker hund. The fat dogs. Aka easy targets. Contrails are unavoidable side effects of high flight. If we could have prevented them, we would have.