Following the Sept. 11th attacks, the Federal Aviation Administration grounded all commercial aircraft in the United States. For three days the skies were nearly as clear and quiet as they’d been since before the Wright Brothers skidded aloft.

These almost empty skies presented scientists with an unprecedented chance to answer a question that has vexed them for years: Do the wispy tracings left behind by jets–known formally as contrails–somehow alter the environment?

A new study in the British journal Nature finds that high-altitude jet contrails have an effect on daily temperatures. The sudden three-day grounding of commercial flights after last September’s terrorist attacks allowed researchers to study the true impacts of contrails from jet trails on climate. The absence of flights eliminated the thin blanket of cirrus clouds that usually forms from the water vapor in the exhaust from jet planes, allowing daytime temperatures to rise and nighttime temperatures to fall.

On a typical travel day, scientists have counted more than 14,000 flights crisscrossing the skies. In the long term, the skies are expected to become even more crowded, making contrails a growing concern for some atmospheric scientists.

Guest:

• David Travis, climatologist at the University of Wisconsin and lead author of a study, published in Nature magazine, on the effect of the grounding of air traffic.