I haven't held forth on this subject to date but I grew up when a "Con Trail", short for condensation trail, could be seen to form and quickly dissipate behind jets. Some time ago a very different phenomenon appeared issuing from some but not all jets. They would produce trails that did not evaporate but seemed to expand into huge diffuse clouds that would cover fast swaths of the sky. There has been much speculation concerning their nature and purpose. Recently some remarkable photos have come forth. Might be another piece of the puzzle. I know those inclined to will immediately discredit the source and thereby the pictures but, for the so inclined, have a gander at this.
http://in5d.com/exposed-photos-from-inside-chemtrail-planes-like-youve-never-seen-before/
http://in5d.com/exposed-photos-from-inside-chemtrail-planes-like-youve-never-seen-before/