Experiments in weather modification are not really that secret, you can read all about them. The Chinese have offices of weather control in each province.
https://en.wikipedia.org/wiki/Beijing_Weather_Modification_Office
this is a pretty good explanation of why you sometimes you can see the vapor trails and sometimes you can't:
https://www.scientificamerican.com/article/why-do-jets-leave-a-white/
RE: Series : Geoengineering | Chemtrails : Goodbye Blue Sky, Here Comes The Chemclouds | Post 4