As I listen to a plane go overhead today, I remember how quiet it was here in Ann Arbor after 9/11 when the entire US airspace was shut down. You could hear the birds a lot more easily, and the sky was clear.
Here's what I wrote at the time:
A fighter jet made a circle over the center of campus as [we] were heading downtown, the only plane in the sky the whole day. On Labor Day there had been a low-flying small plane trailing an advertising banner for a bank (I won't bank there, now) but nothing like that today. Totally clear blue skies and no jet contrails like normal.
From Geotimes, 2004:
The empty skies also allowed David Travis of the University of Wisconsin-Whitewater and his co-workers to determine how contrails might be changing local temperature. Their results, first published in Nature in August 2002 and further elucidated last March in the Journal of Climate, showed that during the days when jets were grounded in September 2001, sunshine increased the temperature at ground level instead of being reflected by contrails. At night, the researchers found, the absence of an insulating layer of contrails meant greater ground level cooling.
It's hard to think that this happened more than a decade ago.