Study Finds Up To 30 Percent Of Americans Believe This Baffling Conspiracy Theory

[ABX]

Plane engines get hot. Much hotter than the atmosphere outside the plane, in fact. So when the exhaust leaves the plane, the water vapor freezes mid-air, causing a trail of ice behind the plane known as "contrails".
Or, if you believe in the "chemtrail" conspiracy theory, they are chemicals deliberately sprayed into the atmosphere by a government agency of your choosing, for unknown purposes......

http://www.iflscience.com/policy/study-finds-up-to-30-percent-of-americans-believe-this-baffling-conspiracy-theory/