Are you worried about chemtrails?

China used weather manipulation to try to cut the pollution out of the air for the Olympic games.

The technology is there. There are private companies even that offer to attempt to manipulate the weather for particular events. I doubt the material is present in normal contrails though. I think it's some kind of aluminum oxide dust that acts as nucleation points for the water.. But I don't really know.

http://www.youtube.com/watch?v=Psdg3OAw_a8&feature=related

There is no such thing as chemtrails. It is just a conspiracy theory put out by people who hate the government.