A few things I've observed about "chemtrail" believers over the past few years:
They seem to have one method of distinguishing a "normal" contrail from a "chemtrail"- persistence. In the chemmie catechism, "normal" contrails dissipate in a matter of minutes, so any contrail which lasts longer, spreads or winds up as part of a newly-formed layer of cirrus clouds must be a chemtrail.
The thing is, this behavior has been observed as far back as WWII, when high-performance aircraft etched the skies of Europe with thousands upon thousands of contrails. I personally remember observing exactly the same thing- a set of five contrails which within a few hours had become identifiable lines in a layer of cirrus- on a nice fall day in 1968. The incident stuck in my memory because I happened to point it out to a classmate who was a black-belt meteorology buff and my, how he did go on about how and why clouds form and dissipate.
If persistence is the diagnostic feature of a "chemtrail", then an inescapable conclusion is that the eevil gummint conspiracy has been spraying us for nearly 40 years at minimum and perhaps for as long as 60 + years. It must not be working very quickly.
Chemmies, however, appear to be completely unaware that the thing which scares them so is by no means a new phenomenon, or of what the implications are for the "persistent contrails = chemtrails" hypothesis.
Anothe thing is that there's real scientific research going on into predicting the formation of persistent contrails with the intent to abate them, and this research isn't at all hard to find. Simply by Googling combinations of the keywords "contrails" "formation" and "persistence" I've turned up really interesting material about the experiental validation of predictive methods by making visual and photographic observations of contrail-forming aircraft, identifying the specific flights involved and their altitudes, obtaining data about weather conditions at those times and altitudes and correlating all that information with what theory predicts would happen.
For some reason, chemmies also seem to be completely oblivious to that work, as well as the history of contrails.
And finally, I can't help but notice that the rationalizations of why the eevil gummint conspiracy is spraying stuff all over the skies have changed dramatically without calling into question the existence of "chemtrails". "Back in the day", as it were, the popular belief was that it was chemical or biological substances intended to make us susceptible to diseases which the NWO would spread for the purpose of reducing the population, or chemicals intended to induce docility. Nowadays, chemmies are more likely to sieze on the fact that there have been some theoretical proposals that dispersing various substances in the atmosphere could be used to counter global warming as evidence in support of ravings about HAARP, weather control and electronic mind control.
It's interesting to see that the absurd inefficiency of using high-flying jets to disperse toxins or mind control chemicals may have sunk in through the chemmies' Alcoa beanies, but more interesting to see that the central belief- that the eevil gummint conspiracy is spraying something on a large scale for presumably evil purposes- retains its "protected from all doubt" status, irrespective of exactly what the evil purpose is claimed to be. This is theological, not scientific, thinking.
No matter what examples of chemical discharges for research purposes or Strangelovian-sounding ideas put forward by some scientists or engineers which conspiracist apologists might dredge up, the salient features of chemmie belief, which are common to most of the popular paranoid conspiracy theories in current circulation, remain. The backwards, start with the conclusion and cherry-pick, hammer and file the evidence to fit it reasoning, the insistence on using the most sinister available interpretation of observations, the circularity, the obliviousness to all information which fails to support or falsifies the PCT... it's all there.
And that, dear friends, is why I consider the conspiracist mode of thought, as a tool for understanding the world we live in, to be about as useful as a rubber screwdriver.