Some of these pictures are indeed simply contrails from passing aircraft, you see the ones behind passenger airplanes, there's a short distance from the engine exhaust where it is clear, then the contrail starts. This is the rapid cooling of the hot exhaust gasses meeting the cooler air at high altitude.

And again with passenger aircraft operating in the upper airspace, there are minimum separation requirements which will often appear like they are flying side by side, but they won't be. A minimum of 1000' vertically and 5NM laterally (IIRC) is the requirement, but it is hard to tell it at such a height.

Now before I get down tooted to death, I do believe in weather modification, it is everywhere. Simply do a search for say "cloud seeding companies in North America" and literally 100's will return.

And yes this goes way back, Vietnam era and before. So whilst I'm with you on this topic, I am also simply pointing out that many pictures in that set are nothing more than contrails of passenger aircraft at high altitude.