Contrails

Contrails (a portmanteau of "condensation" and "trail") are the line-shaped clouds left behind by the engines of aircraft flying at low altitude. They are commonly composed of water in the form of ice crystals.






Chemtrail conspiracy theory

The chemtrail conspiracy theory is an erroneous belief that the contrails of aircraft are being used for nefarious purposes, ranging from altering the weather to mind control. Chemtrail (a portmanteau of "chemical" and "contrail") sightings are often reported on the r/chemtrails sub-messageboard of reddit.com, a discussion website which claimes to be "the front page of the internet".

On this sub-messageboard, under a section titled "Definitions & such" is the short description of a chemtrail as

...a visible trail left in the sky by an aircraft and believed by some to consist of chemical or biological agents released as part of a covert operation.

Following this is a short text that encourages both believers and skeptics to adopt a tolerant attitude towards each other with a disclaimer:

All viewpoints are allowed here, but please be respectful. This is a subreddit, not a court of law. Please don't badger users into providing concrete evidence. Short of flying us all up into the atmosphere with an evidence bag, you probably won't get it.