8 August 2002

Jet trails 'can lead to change in climate'

By David Derbyshire, Science Correspondent

The steady stream of aircraft across our skies is altering the weather, making days cooler and nights warmer, according to research.

A study has shown that vapour trails left by passenger jets reflect sunlight away from the earth during the day, but trap heat at night.

It has long been known that condensation trails, or contrails, left high in the atmosphere can turn into cirrus clouds under the right atmospheric conditions.

But it has been impossible to measure their effect on weather because air traffic never stops, particularly over Europe and North America.

Scientists were given an unexpected chance to tease out the impact of contrails on September 11, 2001, when all commercial planes were grounded in America for three days.

A team from Wisconsin University compared the average daily highest and lowest
temperatures over North America for the flightless days with temperature records
going back to 1977.

They found that the range of temperatures was more than one degree wider if no planes were flying.

The effect usually occurred on a regional scale, at the level of one or two states, said Dr Travis. But in extreme cases, contrail "outbreaks" could cover the entire east or west coasts of America. Contrails are most frequent near areas with the busiest flight paths. The change in temperatures last September was most noticeable in the midwest, north-east and Pacific north-west, he said.

"I have seen many situations where contrails cover more than half the sky with artificial clouds and, in some rare cases, can produce a nearly complete overcast sky," he said.

Satellite images often revealed as many as 50 contrails occurring simultaneously
across the same region, he said. Planes typically produce contrails at cruising altitudes of 30,000 to 40,000ft.

"They are not more likely to occur near busy airport hubs, but instead more likely to occur between the hubs which allow sufficient time for the aircraft to get up to cruising altitudes," he said.

"A single plane would have little effect. Our findings refer to situations where multiple planes are crossing through an atmosphere favourable of supporting persisting contrails across a time span of many hours.

"It is difficult to say exactly how many planes are needed to have an effect, but probably at least 10 and all must be producing contrails."

Dr Travis said similar effects were probably occurring above Britain and Western Europe during days of otherwise clear blue skies.