Chemtrails, for those who doubt, are not really all that secretive.
The US has and private companies have used them for everything from scientific tests to weather modification, and even pest eradication. While it sounds crazy when you just think about it it's rather obvious. Cloud seeding for precipitation has been fairly well known for decades using several chemicals and even liquid propane. Possibly less known are the weather experiments like project Stormfurry, but even that is well documented and publicly acknowledged by the government. The "tin foil hat" really only comes into play when what you think they're "actually" trying to do is something like manipulate the population through chemically induced alterations to public consciousness etc. Those can probably be much more easily dismissed.