Over the next few posts I aim to explore the effects aircraft are having on the environment. I'm going to start with contrails as these are perhaps the most telling signs that a plane is flying overhead, or if a plane has flown by, or if you're really struggling to find one out the window at this point they can be seen in the background picture to this blog! They represent a direct anthropogenic input into the atmosphere. There is extensive literature on them, with some of it being quite contrailversial. Today I am going to look at their formation.
Contrails are linear ice clouds that form behind both propellor and jet aircraft flying in sufficiently cold air (-40 °C), typically found at high altitudes, i.e at the top of the troposphere. They have similar properties and structure to cirrus clouds, which also form at these altitudes. The length of time a contrail lasts for depends on the humidity of the atmosphere the aircraft is flying through. For instance they will be short lived and evaporate quickly (perhaps seconds) when formed in dry air, however more persistent with the possibility of developing into a cirrus cloud layer in air with relative humidity above ice saturation (Schumann et al. 1999).
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| Cirrus Clouds |
In accordance with the Schmidt-Appleman criterion, a contrail is formed when an increase in relative humidity occurs in an engine plume where warm, moist exhaust is expelled from the engine and mixes with surrounding sufficiently cold atmospheric air that enables humidity to reach liquid saturation in the plume (Schumann 2005). The water vapour attaches itself onto condensation nuclei, and in the process freezes into ice crystals almost instantaneously.
Along with the expulsion of mostly water vapour and CO2 from the aircraft engine, nitrogen oxides, carbon monoxide, hydrocarbons, sulphuric oxides, organic material, chemi-ions, soot, and small metal particles from the mechanical erosion of the aircraft are also released, however in much smaller amounts. Some of these provide condensation nuclei which aid in the formation of contrails or increase the affinity of water vapour to them, for example chemo-ions which support the coagulation of small initial ice particles. Even if they are not released however, contrails will still form due to the condensation nuclei already present in the upper troposphere estimated to be around 102 - 104 cm-3 (Schumann 2005).
Despite much research on the formation of contrails, there is comparatively less information on the way that aircraft affect already existing cirrus clouds, and the way in which contrails turn into cirrus clouds when the conditions allow for it. This is due to the microphysical properties of the ice crystals of cirrus clouds being difficult to investigate at high altitudes. Where it was once thought that soot from ageing aircraft were more likely to result in contrails increasing in size to clouds (Schröder et al. 2000), a more recent study concluded that reducing or even eliminating soot in the upper troposphere would still lead to contrail formation (Kärcher and Yu 2009).
The following link is taken from the BBC and shows a series of images of aircraft contrails progressively turning into cloud over the UK.
Finally, the following video shows a contrail coming out of an a380 (the worlds largest passenger airliner!) really well - although difficult to picture, it shows that not all of the contrail is coming out of the aircraft but that the water vapour created in the exhaust plume is also interacting with condensation nuclei already in the atmosphere. The video further emphasises the atmospheric conditions needed for contrail formation, it appears to have been following the aircraft for some time - and that tells us something about relative humidity of the atmosphere.
Stop trying to make contrailversial happen. Its not going to happen.
ReplyDeleteOtherwise a quite interesting post!