About this time of year, every year, the subject seems to pop up in various places. I think
trfour nailed it with this link
http://en.wikipedia.org/wiki/Contrail Take a look at the changes in the upper atmosphere this time of year, dew points are lower, temps are lower and the contrails last much longer. Maybe someone like Rose, who understands the atmosphere better than most of us, could give enlighten us. On clear days the upper atmosphere is dry and contrails persist.
Several years ago there were a lot of stories and letters to the editor in the Conway Daily Sun about contrails and one gentleman in particular was convinced it was a government plot.