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Contrails and cirrus clouds

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Condensation trails, or 'contrails', are the line-shaped clouds that trail behind the back of a plane. They form when water vapour from the engine cools down and condenses. These contrails can last minutes or can grow over time to form 'persistent' contrails that can last hours. If they keep on growing they can form cirrus clouds that last for days or even weeks. Contrails and cirrus clouds contribute to climate change by trapping the Earth's heat in our atmosphere in a similar way to greenhouse gases.

Contrails from a four-engined jet over London, UK. Image: Adrian Pingstone

Scientists are still very uncertain about the full impact of contrails and cirrus clouds on the climate. At the moment it's difficult for experts to predict when a small contrail will disappear quickly, persist in the atmosphere, or grow into a cirrus cloud. It depends on the temperature and the amount of water vapour in the surrounding air. So scientists are tackling the problem head on. By using lots of different approaches they're hoping to get a better understanding of how, when and where contrails are formed.

These contrails are persisting and spreading out. Image: John Flude

One team from Lancaster University have gone back to basics. They've set up a 'contrail observers network' made up of local climate enthusiasts, who are trained to record the number and type of contrails they spot in the sky. The spotters collect their results in the same area and at the same time every day. They record as much detail as they can, including information on whether the contrail is spreading out and forming a cloud or staying pencil thin.

'The most striking result is how much variation there is in the amount of contrails both seasonally and year by year,' says Rob MacKenzie, who heads up the network. 'It's so dependent on general weather patterns, although you do tend to get highs in autumn and spring when the atmosphere's wetter. What we still don't know is how much these local effects over flight paths affect the climate as a whole. We just don't know what happens when you keep on punching the climate in one area, or whether some areas are more sensitive to these effects than others.'

Satellite snaps

Contrails are big enough to be seen from space, so experts are using satellite images over flight paths to track exactly where and when contrails form. They're also busy taking measurements of water vapour, and therefore cloud cover, in areas where planes fly. One team from the University of Leeds are sending up research aircraft that will fly round in circles to measure the conditions after they form a contrail. And a European project is now under way to fit commercial passenger planes with instruments that measure water vapour.

Contrails over Europe as seen from space. Image: NOAA

One team of experts have been studying the effects of contrails after the events of 11 September 2001 in the US. 'When the World Trade Center was demolished all the aircraft were grounded. And over the following 2-3 days we saw some big changes in temperature,' says Piers Forster, a climate expert at the University of Leeds. 'The difference between the daytime maximum temperature and the night-time minimum was 1.5-2 degrees celsius more than usual - a bigger than expected change from contrails and cirrus clouds alone.'

So when the contrails stopped, why did the daytime temperature actually go up? Piers Forster explains. 'This is because although contrails help trap heat in our atmosphere around the clock, in the daytime contrails can also reflect sunlight back to space too, having a small cooling effect on the climate. So without contrails daytime temperatures can actually go up, but night temperatures go down. Overall, the warming effect of contrails is bigger than the cooling effect because warming happens more of the time.'

Piers Forster, Leader of the Physical Climate Change Group, University of Leeds. Image: University of Leeds

So what do the experts currently think? 'At the moment we think the role of contrails and cirrus clouds is roughly comparable to that of carbon dioxide from aviation, but it could be up to 10 times that,' says Piers Forster. 'Aviation currently contributes 2% of global carbon dioxide - which doesn't sound too bad. But if the total effect on the climate is actually many times this it could be really significant.'