I have been reading this discussion for quite some time and felt that I had to add to this discussion because, for once, I see people who are motivated to address a true issue.
One I know a little about.

Point of fact that Skeptical and Blue Sky are both correct in one of their items. Chem Trails are about 6 miles up, or about 20k to 35k feet ... say there are 5k feet to the mile they really are between 4 and 7 miles up there.

Let me explain a little about why this issue is so easy to dismiss.
As a plane flies there is high pressure under the wing and low pressure on top. This is what causes the plane to fly; elementary physics will teach this to you. In my work as an atmospheric chemist I know a little about how a plane flies. At the very tip of the wing - where the high and the low pressure coincide - the air does not know what to do so comes off the wing like water going down the drain - high pressure to low, get it [???]- and if the air is humid water condenses and you see a 'cloud' ... MOST of the lines are from 'trailing edge vortices' ... look it up; you will find it in any advanced aerodynamics text ... I worked for NASA studying this prior to joining the NSA. Part of our work from NASA was to design wing tips for the 747-400 ... go look; you'll see them. We designed them to allow the air to behave cohesively at the tip of a wing.

Chem trails are a real issue. I am an atmospheric scientist working with the NSA. Why, you ask, would the National Security Administration be interested in having atmospheric scientists on their staff ... and why would one of them know anything about chem trails.

You will remember, at the beginning of the Iraq War that there was the worst storm EVER in that part of the world. The NSA is extremely interested in organizing the weather for military operations.

So the skeptics amongst you will ask: why would a Proud Republican Scientist within the NSA be posting classified information on a public web site. Truth be told I feel awful. The weather storms induced in Iraq killed many American Patriots. There are American people who will never see their family members again because of a miscalculation within my office.

Let me close in summary:
Through extensive experimentation and through work we obtained from Soviet sources - who are equally interested in controlling the weather for military purposes - we have developed a method of seeding the upper atmosphere with x-cyanide and x-barium crystals. Atmospheric moisture concentrates around the deposit effecting localized climate change. Through various compositions and mixtures we have the ability to induce rain through severe storms. In Iraq a technician, on site in Dubai, read the mixture wrong and induced a monsoon. The media never reported a monsoon; they reported a sand storm ... if you would never expect a monsoon in Iraq you would never call a storm one ... but look up the data yourself; how much rain did fall, and for how long. Long enough for the atmospheric chemistry to disburse.

Thank you for all your kind work.