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Looking up into the sky, you’ll often see lines of white contrails painted against the blue. Sometimes it’s a mesh, sometimes it’s remarkably parallel, reminding you that there really are highways in the sky. Rarely does a day go by that we don’t see the cloudlike atmospheric footprint of travel in the jet age.

Ever wonder what those jets’ contrails actually do to the atmosphere, and how they affect the weather 35,000 feet below?

For an answer, we turn to to Chicago weatherman extraordinaire, Tom Skilling. For those who don’t know Tom Skilling’s weather reports, they are an exercise in most excellent geekdom. His 9:30 forecasts on WGN go on for far longer, and in far greater scientific detail, than any other local weather forecaster or Weather Channel report I’ve ever seen. We may no longer live in Chicago, but my weather-nerd wife can’t let go of Tom Skilling. His blog remains her daily read. If only his brother, Enron’s disgraced CEO Jeff Skilling, could have followed Tom’s footsteps. At least the older brother made good.

Tom offers this on the contrails question:

The grounding of all U.S. and Canadian commercial air traffic for three days following the Sept. 11, 2001 terrorist attacks constituted a gigantic unplanned experiment, one result of which was a demonstration that high-altitude contrails affect weather at the ground. Contrails, the long, narrow, wispy ice-crystal clouds that form behind high-flying jet aircraft, result from the condensation of water vapor in jet exhaust. It was learned that contrails lower daytime and raise nighttime temperatures slightly in high-traffic jet corridors. Lacking those clouds for a few days after Sept. 11, the spread between daily high and low temperatures increased by about 2 degrees Fahrenheit.

Thanks, Kim!

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