Planes' vapour trails affect weather

Researchers proved jet vapour makes things cooler

By Tim Hirsch BBC environment correspondent

The vapour trails left in the atmosphere by jet aircraft have a significant impact on the weather, according to new research.

Scientists in the United States used the grounding of planes following last year's terrorist attacks to show how temperatures changed when flights were at a standstill.

The strips of cloud left by jets in the sky have long been thought to influence the temperature.

But until now, this has been impossible to measure.

For three days, American skies were empty

The grounding of flights for three days after the 11 September attacks gave scientists in the US a unique opportunity to see what the weather would be like without these vapour trails.

They compared temperatures at 4,000 weather stations across the country with figures for the previous three decades at the same time of year.

Their findings, reported in the journal Nature, show that the gap between daytime and night time temperatures was more than one degree Celsius larger than normal when flights were at a standstill.

This is thought to be because the jet trails reflect sunlight and trap heat.

That makes the day cooler and the night warmer where air traffic is heavy.

However, the effect is almost certainly outweighed by the global warming influence of greenhouse gases, created by the burning of fuel including emissions from jet engines.