Cloud seeding. Already mentioned above. Not in doubt. Not controversial by serious people. But this isn't what SiggotsMeAgain is talking about. Many flatearthers, especially the ones that need you to believe them so badly, often use this as a shoehorn for getting you to agree with them by proxy. I'm sure there's a much better debate term for it. Is it a mott and bailey? I can't remember. Anyway, cloud seeding for rain, more of or less of, is a quite old technique that farmers developed along with scientists for crop growing mainly. That's a different subject in my reckoning, than what is being gotten at when someone is elucidating about "chemtrails".