The length of time a jet trail takes to evaporate varies because of a difference in altitude, humidity, and temperature, ......
True, but that is no the only observation being made. Chemtrails have been well documented. There is a video on youtube about a Vietnam war US airforce pilot who describes how he flew aircraft that sprayed chemicals to change the weather in order to wash out a bridge over several days time. The weather went from clear skies to torrential downpour (during the dry season). That was over 40 years ago.
So when I see "chemtrails are a conspiracy theory" i stop reading.