The first contrails I ever saw were in a movie about Biblical times. Beautiful new Techicolor blue sky, without a cloud, but suddenly a pair of bright white moving lines in the sky. In the early days I guess the propellor driven planes flew lower, and the contrails developed at the wingtips. Now the jets headed to and from LAX make only a single line. I don't know if it is a pair so hight it looks like one or not. And it obviously varies with air temperatures aloft, and relative humidity up there, as it may start suddenly and stop suddenly.
I have seen stories that so many contrails may contribute significantly to global warming. Sounds like bull to me.

this was a topic from my early days on the board:
http://wordsmith.org/board/showflat.pl?Cat=&Board=words&Number=78423
many memories of big South Dakota skies and vapor trails and sonic booms..