Interesting read about the post 9/11 effect. I am suspicious of the Penn State study given the following at the bottom of the linked article:
"The National Science Foundation supported this work."
I think they have an agenda.
"A Penn State study
compared regions of the United States where contrails tended to form more strongly with areas where they didn’t. The more contrail-heavy the area, the less the variation between daytime highs and nighttime lows tended to be."