The grounding of commercial flights for three days after last September's terrorist attacks in the United States gave David Travis at the University of Wisconsin-Whitewater and colleagues a chance they never thought they'd have: to study the true impact that contrails from jet engines have on our climate1.



Travis's team compared the average daily high and low temperatures over North America from 11 to 14 September 2001, with climatic records from 1977 to 2000, matching the weather over those three days with similar weather in September from historical records.

They found that the difference between daily high and nightly low temperatures in the absence of contrails was more than 1 oC greater than in the presence of contrails. Comparing the three-day grounding period with the three days immediately before and after, the impact was even larger - about 1.8 oC.

"It's obviously a significant effect," says Andrew Carleton, an atmospheric scientist at Pennsylvania State University in University Park and a member of Travis's team.

The researchers suggest that in regions with crowded skies, contrails work just like artificial cirrus clouds, preventing days from getting too hot by reflecting the Sun's rays, and keeping nights warmer by trapping the Earth's heat [link to www.nature.com]