The weather on 9/10/01 and 9/11/01 was absolutely THE most perfect days I've ever seen in my flying career.  I started to wonder about weather modification after that.  Too many things happened at that time that made me go, hhhhmmmm. About a year before that, I was flying with a student when I flew under a cloud that stretched from horizon to horizon and cast a deep, dark, and sharp line on the ground.  It was a rather strange thing at the time, and mentally noted it, putting it in the back of my mind for later.