Dear Tom,

I remember reading that the flight ban following the September 11 attacks affected the weather. What was that all about?

-Dee Callahan

Dear Dee,

All commercial aircraft across the United States and Canada were grounded for three days following the terrorist attacks of September 11, 2001. That constituted a gigantic unplanned weather experiment, one consequence of which was a demonstration that high-altitude jet condensation trails affect surface temperatures. Water vapor in jet exhaust condenses into a thin veil of ice-crystal clouds that lowers daytime and raises nighttime temperatures slightly in very high-traffic jet corridors.
In the absence of those clouds during the three days of grounded jet traffic, the spread between daily high and low temperatures increased by 2 degrees.