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 -- that is, the difference between the daytime maximum and the night-time minimum -- was found to be more than 1c more than when contrails are present. In other words, the contrails had an insulating effect on the daily temperatures, reducing the variation between day and night.

These findings corroborate previous studies conducted in western Europe, which suggested that the contrail effect is at least as important as global warming as the effect of greenhouse gases like carbon dioxide. However, Carleton says it is still extremely difficult to predict whether contrails exacerbate global warming or counteract it.

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