We've all seen those wispy white trails that aircraft leave in the skies. Until now, scientists have been unable to calculate the impact on weather of these vapour trails. That is until post 9/11.

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 with no air traffic at all.

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 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.