11/03/2010

Have you noticed those expanding jet aircraft vapour trails?

We all see the vapour trails that jet aircraft can leave in a clear blue sky. Sometimes they disappear rapidly, sometimes less rapidly, sometimes they just keep expanding. Explanations!


vapour 002.JPGI thought about these vapour trails when I was in the Valais mountains last week. There were not too many of them  (their proper name is contrails), so even when they did not immediately disappear, they left the sky as very blue. However, when I came back to Geneva in the train, as I got to Cornavin station I noticed a very much whiter sky, where the vapour trails just seemed to have kept expanding. This photo shows you what I saw.

After I did a bit of an investigation I learned that vapour trails happen when aircraft fly above about 25000 feet, in a temperature of around -30° C, and the water vapour that the engines emit freezes into visible ice crystals. If the air up there is dry then these do not last long in the sunlight, but if the air is moist then the crystals act as a catalyst for more water condensation and freezing, eventually causing the the formation of cirrus clouds.

All of this is what environmentalists cite as one of the negative aspects of the increasing air traffic: it can happen even when we are not seeing the  blue skies. Geneva, with its considerable transit traffic high above us, is much more exposed than the Valais region, whilst London is even more susceptible to having a blue sky turn white. In fact, at the time of the ash cloud crisis, there were many newspaper articles saying how blue was the sky without the aircraft!

The possible scale of the problem was realised last year, when UK scientists used a weather satellite to observe the effect of a large military aircraft circling over the North sea. They were astonished to find that instead of dispersing in the high-level winds, the contrails steadily expanded, eventually causing clouds covering 20,000 square miles (50,000 square kilometres). That is more than the area of Switzerland!

It has been suggested that if the aircraft flew a little lower, this effect could be avoided, but that would be less efficient (more fuel needed, therefore more immediate cost). In other words, a non-starter.

So, when you fly to your sunny holiday destination, you may be contributing to less sunshine for people over whom you fly!

 

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