Edward Teller wrote a paper 9 years ago on the benefits of spraying reflective particulants to counter regional excessive warming of the atmosphere.

The phenomena is only 7 years old but is quite real.
That commercial jets have changed their fuel to JP7 may or may not have something to do with it.

In DC we have had many of these so called chem trails.

Here is what one looked like from our yard, north of DC 6 days after 9-11. (Note all commercial jets at National airport were grounded at the time.)
http://www.angelfire.com/md2/customviolins/chem.jpg