Are "chemtrails" real?

Someone I know is convinced chemtrails are a dire threat. I agreed to dig into the matter. This page is for that purpose.

At this point the evidence is overwhelmingly that they don't exist.

References

1. Why I Write About (and Debunk) the Chemtrail Conspiracy Theory
2. Wikipedia page
3. GeoEngineeringWatch.org
4. Chemtrails Conspiracy Theory
5. First Published Study on 'Chemtrails' Finds No Evidence of a Cover-Up