Rain Falls Mainly by the Plane

01 July 2011
Posted by Chris Smith.

Aeroplanes punching their way through clouds near major airports can increase local precipitation.

A paper published in Science this week shows how the passage of aircraft through super-cooled clouds can form distinctive holes and canals.  Andrew Heymsfield and colleagues, based at the National Center for Atmospheric Research in Colorado, used satellite data and modelling to discover the major factors contributing to the holes.

Super-cooled clouds contain water droplets that are still liquid despite the temperature being below their freezing point, and remain in this state down to -40 °C.  Any colder than this and the water droplets tend to form ice crystals.

A plane wingCounter-intuitively, the passage of planes through such clouds, rather than warming them, can cause a drop in temperature by as much as 20-30 °C.  This drop is caused by expansion of air behind the propeller tips and air flow over the wings, and is often enough to form ice crystals which can then grow rapidly.  These ice crystals can become so large that they fall out of the cloud as precipitation, commonly snow, leaving behind holes and voids in the cloud.  

These unusual cloud patterns have been observed for a long time from the ground and using satellites, but this recent paper sheds more light on the process.  Many types of plane can cause hole formation to occur, including passenger and military jets.  The effect is more pronounced near the poles, suggesting that data for climate modellers based in the poles may be skewed by their proximity to aircraft landing sites.  It is also something that cloud scientists should be wary of, as they need to make sure they are not taking measurements from areas of the cloud that they have flown through and disrupted.

By analysing data that was collected over a long time-scale, Heymsfield has shown that clouds around airports within a 100km radius are being altered 5-6% of the time. This is unlikely to have a global contribution to climate but could certainly increase the amount of snowfall around major airports, possibly leading to more demand for de-icing in the future.

So next time you are jetting off to sunny climes, spare a thought for the rain and snow you leave behind.

Interview with Greg Thompson from the National Center for Atmospheric Research in Colorado talking about airplanes affecting clouds.

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