The sky of 9/11

On this past Monday, the fifth anniversary of 9/11, I noticed the sky was clear and blue. But the blue was an ordinary blue, nothing like the intense clarity of the sky five years ago. It's one of the things many people remember best about 9/11: the piercingly blue sky, so unusual in its intensity as to make the untouched photos seem Photoshopped.

I also recall that the day before 9/11, and for a day or two afterwards, the sky retained that unusual sharpness and essence of blue. I'd never seen it before, nor have I since. I wondered whether the hijackers considered it an omen, a sign that their mission had the approval of the deity.

I also seem to remember reading somewhere about what had caused the unusual sky, and an aviation term to refer to it. But now I can't find a thing about it. So, dear readers: can anyone help out with this?