This is a very interesting article about how air travel is affecting the weather. The ban on air travel on Sept. 11 let researchers see what the constant jet traffic is doing to the environment. Specifically, jet contrails tend to create high cirrus clouds that can trap heat.
(note: NYT requires registration before viewing)
While it doesn't address it specifically, I was thinking about how this could be affecting astronomical viewing. I was shocked to see how many high clouds were created by jet contrails. This must be affecting the clarity of the skies. Perhaps some of the haziness a lot of people have to contend with is caused by air traffic?