From: PBS.org—Dimming the Sun
At least that was the case until September 11, 2001. For the first time since the jet age began, virtually all aircraft were grounded over the United States for three days. Even as they tried like the rest of us to absorb the enormity of the terrorist attacks, climatologists realized they had an unprecedented opportunity to scrutinize individual contrails, and several studies were quickly launched. …
Another study that took advantage of the grounding gave striking evidence of what contrails can do. David Travis of the University of Wisconsin-Whitewater and two colleagues measured the difference, over those three contrail-free days, between the highest daytime temperature and the lowest nighttime temperature across the continental U.S. They compared those data with the average range in day-night temperatures for the period 1971-2000, again across the contiguous 48 states. Travis’s team discovered that from roughly midday September 11 to midday September 14, the days had become warmer and the nights cooler, with the overall range greater by about two degrees Fahrenheit.