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.