Stream of consciousness: this thread made me think of my uncle, who worked to develop cloud seeding with Bernie Vonnegut and a couple of others, and I remembered I had a picture of Bernie (taken by said uncle) as a young man with his son and Googled him. Sent a query to a guy who had a memorial page for him. Found another interesting page on Cloud Physicist/Meteorologist Robert M. Cunningham, in which he writes,

I also recall a meeting in Washington on what the federal government should be doing in the weather modification field (also in the 60's?). This was an informal meeting of about 6 people including science administrators and scientists, principally from the defense science branches and the National Science Foundation. I was surprised when Dr Edward Teller walked into the room. He was invited just because he was interested in weather modification; perhaps he was looking for an area that could explode with world interest as had his previous project. I was, however, disappointed with what he said; he followed Langmuir's belief that simply burning a torch with a silver iodide solution in the fuel could change the whole world's weather. He had no appreciation of the vastly different scale over which weather systems occur.

So Teller vastly overestimated the effect a small-scale activity could have on the environment. It seems to me this thread does the same. Even if those planes were engaged in spraying stuff and weren't just going about their mundane business, it's one geographically small city on a globe full of mostly empty sky.