I've posted it here before, but I hadn't come across this particular article.
The Contrail Effect
Quote:
For the first time since the jet age began, virtually all aircraft were grounded over the United States for three days. Even as they tried like the rest of us to absorb the enormity of the terrorist attacks, climatologists realized they had an unprecedented opportunity to scrutinize individual contrails, and several studies were quickly launched.
One study looked at the aforementioned contrails that grew to cover 7,700 square miles. Those condensation trails arose in the wake of six military aircraft flying between Virginia and Pennsylvania on September 12, 2001.
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They've moved on to study the effects of cloud coverage on temperatures. Take it or leave it, but it's pretty well established that planes create cloud cover.