THE clouds formed by aeroplane exhaust trails are affecting the temperatures we feel on Earth.
鈥淐ontrails鈥�, the trails of water vapour left by jet planes, form artificial cirrus clouds under the right atmospheric conditions. Natural cirrus clouds protect the planet from extremes of temperature by reflecting sunlight from above and absorbing heat from below.
It has been difficult to determine whether contrails affect climate in the same way because air traffic is so constant. But the three-day grounding of aeroplanes across North America after September 11 gave David Travis and his colleagues at the University of Wisconsin the chance they needed to find out.
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They compared the average daily maximum and minimum temperatures across North America鈥檚 jet-free skies with those for the same period from 1971 to 2000. The range of temperatures they measured was bigger by more than 1掳 C, suggesting that contrails add to the insulating effect of cirrus clouds on our climate (Nature, vol 418, p 601). Calling the tunes