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Contrails affect temperatures©Associated PressAugust 9, 2002 The wispy trails of exhaust that jetliners spew high in the sky form cirrus clouds that have a small but potentially important effect on ground temperatures, scientists report. When commercial jet traffic was halted by last fall's terrorist attacks, researchers seized upon the unique opportunity to compare the climate data for the clear skies from Sept. 11 to 14 against days of normal air traffic when jets streak the heavens with contrails. Their conclusion: Without jets or contrails, the clear skies boosted the temperature swing between daytime highs and nighttime lows by about 3 degrees. The temperature swing in the Northeast and Midwest, where there is more jet traffic, rose as much as 5 degrees. In the South and Southwest it was smaller, partly because the warmer, drier climate produces fewer clouds. How all this figures into global warming is unclear and something that needs to be studied, said climatologist David Travis of the University of Wisconsin at Whitewater, lead author of the study appearing in the current issue of the journal Nature. Also unknown is whether it might influence agriculture by changing crop growth and the emergence of pests and diseases. The study compared only the range of temperature swings between day and night, also known as the diurnal cycle. The researchers did not compare actual temperatures recorded during each period of the days under consideration. Researchers said they suspect that the jet-spawned clouds are lowering temperatures during the day and boosting nighttime readings, but more research is needed.
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