Changes in the weather in the days following the September 11 terrorist attacks

Weather Blog

Dear Tom,

I remember hearing about some changes in the weather in the days following the September 11 terrorist attacks. What happened?

Thanks,  
Charlie S.
DeKalb

Dear Charlie,

Following the September 11, 2001 terrorist attacks, all U.S. and Canadian commercial aircraft traffic was grounded for three days. As a result, there were no high-altitude jet condensation trails in the skies. These trails form when water vapor in the jet’s exhaust condenses into a thin veil of ice-crystal clouds that fan out and spread across the sky, often forming a cirrus shield. With a small increase in both incoming and outgoing radiation, the effect of their absence was to slightly raise daytime and lower nighttime temperatures in high-traffic jet corridors. As a result, during the three-day flight stoppage, the spread between the daily high and low temperatures increased by about two degrees.

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