Iridescent ice clouds from aircraft wings
Not all condensation trails are the same. Scientists from the DLR Institute of Atmospheric Physics have, for the first time, been able to describe how condensation trails are created not only from the exhaust gas of jet engines but, under certain conditions, also by the wings of aeroplanes. It is unclear to what extent this new class of condensation trail influences climate change, as up until now it has not been considered in climate models.
The cause of the condensation trails, which are initially very thin and can only be observed directly behind an aeroplane at cruising altitude under certain conditions, is the rapid decrease in pressure above the wings of an aeroplane. Due to the drop in pressure, the temperature of the air falls in a few hundredths of a second and, provided that there is sufficient moisture in the air, tiny ice particles are formed. Scientists from the DLR Institute of Atmospheric Physics (Institut für Physik der Atmosphäre) in Oberpfaffenhofen have, for the first time, been able to conclusively describe the formation mechanism for aerodynamic condensation trails.
The rainbow colours of this type of condensation trails are of particular interest. At a distance of around 50 metres behind the aeroplane the ice particles are still small and they reflect blue light in particular; the larger the ice particles become – that is, the further they are from the aeroplane – the longer the wavelength of the light they reflect. The colour of the condensation cloud thus changes from blue to green to yellow and finally enters the red part of spectrum. After around 175 metres, the condensation trail appears white because the ice particles are drawn, little by little, into the turbulence of the wake behind the aeroplane. Here, the ice particles no longer grow evenly; due to their different sizes they reflect the light diffusely at all wavelengths and therefore appear white.
All condensation trails caused by high-flying aeroplanes are ice clouds, of a type known as cirrus. They increase the degree of cloud coverage and change the natural cirrus clouds, thus influencing the climate. Aerodynamic condensation trails represent an additional anthropogenic source of cloud that contributes to the effect of air traffic on the climate and which to date has not been taken into account. The researchers' goal is to integrate aerodynamic condensation trails into a climate model that determines the global effect of air traffic on climate in order to identify the role it plays in climate change. Within the framework of DLR's 'environmentally friendly air transport system' project, the researchers are also looking for approaches to enable the effect of air traffic on the climate to be minimised, for example through appropriate air routes or aeroplane designs.