In article .com,
Nelson wrote:

Aircraft leave vapour trails which aid in the reflection of
the sun's radiation back into space.

A study of the no-fly period following 9/11 showed a full extra degree
(Celsius) of cooling over the US during those four days, apparently due to
the lack of contrail vapor, which contributes to the cirrus cloud layer that
reflects nighttime longwave radiation back to earth.

--
Randy Hudson