From time to time, beautiful clouds show up in the sky. They are called nacreous clouds (from nacre, mother of pearl), due to their beautiful iridescent colour, and are located high up in the atmosphere. But did you know that these pearly clouds often further the depletion of the ozone layer?
By senior scientist Tove M. Svendby, is employed at NILU’s Department of Atmosphere and Climate
Nacreous clouds are a form of so-called PSCs (Polar Stratospheric Clouds), located in heights of 15-30 km (stratosphere). This is approximately twice as high as ordinary ice clouds in the troposphere (up to 15 km). They are usually seen during the winter in Polar Regions. Since nacreous clouds are found at such high altitudes, sunlight hits them from below during sunrise and/or sunset. The ice crystals break the sunbeams like a prism, resulting in an impressive spectrum of colours.
Systematic ozone measurements started in Norway back in the 1930s, first in Tromsø and some years later at Dombås. Meteorologist Kaare Langlo, responsible for the Norwegian ozone measurements in the 1940s, discovered that the formation of nacreous clouds was often connected with low ozone levels.
Langlo did not find a good explanation of this phenomenon, but concluded in a publication in 1952: «We have found justifications for publishing these preliminary results in the hope that they may be of some value for continued research in this field». Langlo’s observations were the result of “heterogeneous ozone depletion”, but several decades would pass before this phenomenon was scientifically explained.
Nacreous clouds + chlorine = ozone holes
The formation of nacreous clouds requires low temperatures (below -80°C) in the stratosphere. Such ice clouds occur fairly rarely over the Arctic, whereas they are considerably more common over the Antarctic. They are also an important reason for the massive depletion of the ozone layer (ozone hole) which occurs each spring over the Antarctic.
The atmosphere contains both natural and anthropogenic chlorine compounds. Our use of CFC-gases (chlorofluorocarbon, i.a. from spray cans) has contributed to a significant increase of chlorine compounds in the stratosphere over the last decades. Chlorine compounds are initially stable and react with ozone only to a small degree. However, chemical reactions between passive chlorine compounds on the surface of PSCs (nacreous clouds) lead to the formation of chlorine gas (Cl2).
Darkness covers Antarctica between June and August. But as soon as the sun peeks out in September, the sunlight splits Cl2 into Cl radicals that attack and break down ozone. At the same time, the polar vortex over the Antarctic (a persistent, large-scale cyclone, circling the Polar Regions and reaching from the troposphere into the stratosphere) prevents the ozone rich air from lower latitudes reaching the Antarctic. Thus, the ozone hole is a fact. At the end of November, the polar vortex usually breaks up, and the ozone hole once again fills up with ozone rich air from the equator.
Smaller ozone hole over the Arctic than over the Antarctic
In order to form stratospheric ice clouds (nacreous clouds), it must be extremely cold in the stratosphere. Due to topographic differences in the Northern and Southern hemispheres, the stratosphere is usually warmer over the Arctic than over the Antarctic, and the polar vortex there less stable. Because of this, there rarely occur as powerful and long lasting ozone holes over the Northern hemisphere. The winter and spring of 2011 was an exception, however – at the time a long lasting ozone hole was observed also over the Northern areas. This was due to an unusually cold stratosphere over a period of several weeks, in addition to a low supply of ozone rich air from the Southern latitudes.
Nacreous clouds and climate
Due to increasing CO2-concentrations, the troposphere will warm up, whereas the temperature in the stratosphere is expected to decrease. This may lead to a more frequent occurrence of nacreous clouds, and to ozone holes becoming more common over the Arctic. The climatic interactions are many, and future mother of pearl clouds are difficult to predict.
Ozone hole or not – these colourful clouds are an impressive spectacle followed with great interest by both scientists and others with a sense of beautiful natural phenomena.