Dr. Weeks’ Comment: A contrail is a trail on condensation (con-trail) which appears in the wake of an airplane based upon the temperature differential between engine exhaust (hot) and high altitude way (cold). We have all seen contrails and we note that these trails dissipate rapidly as the temperature differential equalizes. That is why contrails disappear at what appears to be about 2 inches behind the airplane (see the visual comparison starting at 1:20 in the video below).
A chemtrail (chemicals being dumped from airplane) persist and bloom and spread and persist for hours in the air. This is not a condensation effect. It is a chemical spraying. Look up and get interested.
Here is an excellent introductory you tube video