There has been such a thing as cloud seeding around for over a century now. That is generally small and local, and as far as I know rarely if ever used anymore. Just off the top of my head, I think the chem trails that people are seeing are often just condensation in the form of ice crystals from the hot jet exhaust and cold air the jet if flying in. At 30 thousand feet the temperature even in the summer would be near zero or below zero. Sometimes they evaporate quickly, sometimes the would likely linger in the form of "trails" if the upper atmosphere is moist enough.