What are the white trailers you see behind airplanes in a blue sky?

exhaust

contrails or vapor trails

those are called contrails!

That’s a great word. They’re trails of condensed water—condensation trails—made by the water vapor coming off the plane’s engines.

When I was a child – more years ago than I care to disclose – I asked my father and I thought said they were “paper trails”. I had a vision of guys shoveling reams of white waste paper out the back of the plane. I had no idea why, but I was just a little kid…

just throwing out a wild conspiracy theory here… http://weatherwars.info/

if nothing else, there are several topics there that make for interesting reading. fun to freak out your friends and coworkers with the craziness.

sorry, if you do look at http://weatherwars.info/, look for the links about “chemtrails”... which is what I intended to say in direct answer to your question.