It's weather modification, mostly. I met a pilot in New Mexico who told me that he was contracted to "release" or "seed" clouds to create thunderstorms to combat a severe drought. The mixture is highly classified, of course, and even telling me this was more of a "nod" to my questions than an outright statement because of his strict NDA. If you call those efforts chemtrails then yes, it's been happening for a long time and not all of it is classified.