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Offshoots

Offshoots recently, us researchers have proved for the first time that the water vapour exhaust trails or contrails that are left by cruising jet planes have an insulating effect on daily temperatures. Andrew Carleton from us-based Pennsylvania State University and colleagues measured the effect of contrails during the three-day grounding of commercial aircraft after September 11 last year, when the terrorist attacks on the World Trade Centre and the Pentagon occurred.

Though the circumstances were tragic, the lack of air traffic granted a unique opportunity to test the impact of contrails. Previously it was virtually impossible to measure their effect on climate because air traffic, especially over regions like Europe and North America, never stopped.

The researchers compared the average daily highest and lowest temperatures over North America for this period with records for the same interval between 1971 and 2000. In the absence of contrails, the range of temperatures

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