[/quote]You must have seen a different version of the programme to the one I saw. Water vapour is a greenhouse gas, and the research also showed that the contrails at altitude contribute significantly to the greenhouse effect. The
net effect of a contrail is an
increase in overall global warming. The results that I heard seemed to show that global warming may have been
slowed down as a result of the decreased air movements after 11/9. You are right about the night-time effect, though - there is only the 'blacket effect' at night, when there is no sunlight to reflect, so the effect of night-flights is worse.[/quote]
I was a different programme Stu it was Horizon now I think of it:
Here's the link:
http://www.bbc.co.uk/sn/tvradio/program ... rans.shtml
THis is the main bit, basically the water droplets reflect a portion of the suns ray's back up, hence reducing global warming
NARRATOR: It was David Travis who first caught a glimpse of what the world could be like without Global Dimming. It happened in those chaotic days following the tragedy of 9/11. For fifteen years, Travis had been studying the vapour trails, or contrails, left behind by high-flying aircraft. Though each individual contrail seems small, when they all spread out, they can blanket the sky.
DR DAVID TRAVIS: Here are some examples of what we call outbreaks of contrails. These are large clusters of contrails. And here's a particularly er good one from Southern California. Here's the west coast of the United States. And you can see here this lacing network of contrails er covering at least fifty per cent, if not seventy five per cent or more of the sky in that area. It doesn't take an expert to er realise that if, if you look at the satellite picture and see this kind of contrail coverage that they've got to be having an effect on temperature at the surface.
NARRATOR: But the problem Travis faced was to establish exactly how big an effect the contrails were actually having. The only way to do that was to find a period of time when, although conditions were right for contrails to form, there were no flights. And, of course, that never happened. Until September 2001. Then, for three days after the 11th virtually all commercial aircraft in the US were grounded. It was an opportunity Travis could not afford to miss. He set about gathering temperature records from all over the USA.
DR DAVID TRAVIS: Initially data from over 5,000 weather stations across the 48 united states, the areas that was most dominantly affected by the grounding.
NARRATOR: Travis was not looking just at temperature - that varies a lot from day to day anyway. Instead he focused on something that normally only changes quite slowly: the temperature range. The difference between the highest temperature during the day and the lowest at night. Had this changed at all during the three days of the grounding?
DR DAVID TRAVIS: As we began to look at the climate data and the evidence began to grow I got more and more excited. The actual results were much larger than I expected. So here we see for the 3 days preceding September 11th a slightly negative value of temperature range with lots of contrails as normal. Then we have this sudden spike right here of the 3 day period. This reflects lack of clouds, lack of contrails, warmer days cooler nights, exactly what we expected but even larger than what we expected. So what this indicates is that during this 3 day period we had a sudden drop in Global Dimming contributed from airplanes.
NARRATOR: During the grounding the temperature range jumped by over a degree Celsius. Travis had never seen anything like it before.
DR DAVID TRAVIS: This was the largest temperature swing of this magnitude in the last thirty years.
NARRATOR: If so much could happen in such a short time, removing just one form of pollution, then it suggests that the overall effect of Global Dimming on world temperatures could be huge.
