Yahoo is running this news story about the effects of contrails (condensation trails formed by jets passing through the air) on local temperature variation.  The long-held belief was that contrails, which often create cirrus clouds that remain for long periods of time, reduced daytime temperatures in local areas and reduced nighttime cooling.  Scientists compared the local temperatures in selected areas during the four days following September 11 (when there was no commercial air traffic) against the averages, and found that there is an indication that the theory is correct:  contrails are causing local temperature variations.  The Yahoo story (grabbed from AP) is a pretty poor summation of the research, by the way.  A Wired story from May 15 is a bit better, but you’re probably better off finding the article in the next issue of Nature.  Especially bad was an interview on NPR (I think it was Morning Edition) where somebody characterized the results as providing “irrefutable evidence”.  There are plenty of grounds on which to refute the evidence.

Contrails, by the way, are water vapor—a by-product of burning jet fuel.  Contrary to popularly held belief, they are not “exhaust” in the sense of soot or other noxious chemicals.  The cold atmosphere at those altitudes (25,000 to 45,000 feet, usually) won’t hold the water vapor, so it condenses to form clouds.  A good place to start learning about contrails is this article over at HowStuffWorks.