The three-day interruption of commercial jet aircraft trafic over the entire United States, after the September 11 terrorist attacks, has led climatologists to a surprising scientific finding. They discovered that the moist exhaust trails that come out of the back of jets, known as jet condensation trails or "contrails", reduce the temperature on the ground.

Within hours of the terrorist attacks on New York City's World Trade Center and the Pentagon outside Washington, D.C. on September 11, all commercial and private planes in the United States were grounded for three days.

The absence of almost all jet traffic created a condition over the entire country that gave researcher David Travis and colleagues at the University of Wisconsin at Whitewater a unique opportunity for study.

They were able to judge the effect jet contrails, the wispy, cirrus-like clouds, formed high in the atmosphere, have on ground temperatures.

"We did find that the temperature range did have an unusually large increase during that three-day period," he said. When we compared it to a three-day period immediately before the terrorist attacks and a three-day period immediately following the return of the commerical aircraft to the skies on the 14th (of September)."

The researchers discovered that without contrails, temperatures in the lower 48 United States are more than one degree Celsius warmer. The findings are reported in the current issue of the journal Nature.

One question is what impact do jet contrails have on global warming?

Professor Travis says contrails are not like greenhouse gases, which trap the sun's heat within the earth's upper atmosphere. "Jet contrails affect regional areas, say the size of a couple of states. And they're only there temporary. They're only there for a few hours," he says. "We get a cluster of contrails and then they dissipate. So, we're operating at totally different scales. That really complicates the global warming debate."

The reason, while jet contrails can cool ground temperatures during the day, they can substantially warm the atmosphere at night, according to David Travis. Professor Travis says he and his colleagues plan to investigate which contrail effect dominates, daytime cooling or nighttime warming.

Mr. Travis adds contrails can affect local temperatures anywhere in the world where there is heavy jetliner traffic and atmospheric conditions that are ripe for cloud formation.