Jet trails occur rather naturally when the exhaust of a jet plane meets with the atmosphere.
"It’s a reaction of the atmosphere to the jet exhaust being let out," Richard Castro, National Weather Service meteorologist said. "They're basically artificial clouds.
"They're triggered by water vapor in the exhaust of the aircraft engines."
Castro said a lot of good information on contrails can be found here.
"They almost actually become clouds, almost," Castro said.