That, my friends, is the trillion dollar question.
The video posted below is as thorough an exploration of Chemtrails as I can find in the modern mainstream media. It doesn't refute the existence of Chemtrails -- in fact it confirms that massive Geoengineering programs are underway around the globe. The video also reveals how many unanswered questions present themselves to anyone who approaches this well-documented and rather disturbing modern phenomenon.
RT News, Friday September 9th 2011:
The American mainstream media doesn't touch the subject very much anymore, but a few years ago it popped up on mainstream newscasts across the country: now we hear next to nothing from the mainstream while more planes are visibly spraying every day. Watch the first few minutes of the following video for a smattering of what American coverage used to look like.
If you want irrefutable evidence that Chemtrails truly exist and a lot of credible info on the topic, open your mind and watch the brilliant documentary "What In The World Are They Spraying?" It's 1:37 long, but well worth the time commitment.
Unfortunately, the true answer to the question "What in the world are they spraying?" is...
WHATEVER THEY WANT, BECAUSE THERE IS ZERO ACCOUNTABILITY.
If you're willing to go all the way down the Chemtrail Rabbit Hole, this is a good place to start your search. This video meshes with my own research, and I encourage you to conduct your own.