Polare Stratosphärenwolken

Type 2 PSCs consist of pure water ice crystals. They form at lower temperatures, namely at -85°C to -90°C and below (188 K at 25 km altitude). Their particle size is approximately 10 µm. The ice crystals are so heavy that these PSCs tend to sink into the troposphere. The already very water-poor stratosphere becomes further dehydrated over the poles. This type of PSC, also called nacreous clouds, usually has a lenticular appearance and occurs only in small areas.

The following photos were taken by Hákon Hansson from and in Iceland. We thank you for these great pictures.

Linda Ólafsdóttir also captured these colorful clouds on January 26, 2012 in Iceland.

Nacreous clouds are described as usually pastel-colored iridescence on the smallest ice crystals of lenticular clouds at an altitude of 20 to 30 km. They are best visible shortly before sunset or just after sunrise, at a distance of 10 to 20° from the sun. However, they can also be observed up to 2 hours after sunset, indicating that they are located at high altitudes. They form when an airflow moves over an obstacle, such as a mountain range. This causes the airflow to oscillate, and on the leeward side, stationary waves form in stable atmospheric layers. In these lee waves, the air alternately flows upward and downward. In the sections with upward movement, the air expands and cools down. This allows water vapor to condense and clouds to form. In the northern latitudes, wave formation extends to the highest layers of the atmosphere due to the extremely stable atmospheric structure. However, since temperatures rarely drop low enough there, nacreous clouds only form occasionally. In the Arctic and Antarctic, they are more frequent in winter than previously thought, according to recent knowledge. It is assumed that dust in the stratosphere promotes the formation of nacreous clouds, as tiny dust particles serve well as sublimation nuclei for water molecules. In Scandinavia, nacreous clouds can be observed almost every winter. Finnish observers can look back on over 50 occurrences in 12 years thanks to the Scandinavian Mountains blocking the westerly winds. Whether nacreous clouds are also possible in Germany has so far only been speculated. The theoretical conditions are occasionally present in winter in our latitudes, especially in northern Germany, where the climate of the higher layers of the atmosphere is still influenced by the Scandinavian Mountains. However, there are hardly any observation reports from Germany. The literature only mentions one such case. The German "Astronomical Notices" of 1910 reported that such clouds were observed on May 19, 1910, shortly after Halley's Comet passed. We would be very grateful for further literary references.

On December 1, 1999, Heino Bardenhagen observed in Helvesiek shortly after sunrise a sky that gave the impression of a wavy water surface, reflecting sunlight dimly. The "pond" seemed to be upside down. This observation is likely about other stratospheric clouds. At just 75°C, for example, nitric acid (HNO3), which is present in small amounts in the atmosphere, can condense. From this, very thin, fibrous-looking cloud fields often stretching over thousands of kilometers can form. Since similar clouds were observed over a large area from central Scandinavia to northern Germany that night, these observations could also be of the so-called Nitric Acid Trihydrate Clouds (NAT). The Institutes for Environmental Physics of the Universities of Heidelberg and Bremen and the Norwegian Institute for Air Research reported the following in their ozone bulletin for the corresponding period: While the stratosphere was comparatively warm last winter and very low chlorine activation could be measured, the stratosphere cooled rapidly at the end of 1999, allowing the formation of large-scale polar stratospheric clouds from mid-December. Polar stratospheric clouds have already been observed in large numbers from various ground stations. Meteorological temperature analyses from January 2000 show that in the year, at an altitude of 20 km, areas with cold temperatures below 195 K (-78°C) in the Northern Hemisphere had never been as large as they were then.

The occurrence of these low temperatures is related to the extreme conditions of the polar regions because the air masses over the poles are completely isolated from the usual global air currents in winter. As soon as the sun disappears below the horizon for several months in late autumn, an intense westerly flow forms around the pole, the so-called polar vortex. This polar vortex forms a ring-shaped current and hinders the air exchange with the rest of the atmosphere. Only then can stratospheric temperatures in this area fall to such low values. The polar vortices are particularly developed in Antarctica, which is related to the large landmasses at the South Pole. The vortices over the Arctic and the processes associated with them are generally less pronounced.

It was predicted that there will be a record ozone depletion over the poles in the coming months, because according to the latest scientific findings, stratospheric clouds play a major role in ozone depletion. Under normal conditions, the chlorine originating from released CFCs is fortunately mostly bound in the so-called chlorine reservoirs. These are substances that do contain chlorine atoms but do not contribute to ozone depletion. The most important chlorine reservoirs are hydrochloric acid (CHI) and chlorine nitrate (ClONO2). Hydrochloric acid is formed from the reaction of chlorine (Cl) with methane (CH4). Chlorine nitrate forms from chlorine monoxide (ClO) and nitrogen dioxide (NO2). Without these two substances, which bind almost all the chlorine in the atmosphere, much more ozone would be depleted in the atmosphere than is actually the case. The ozone hole is therefore created according to current knowledge because under the special conditions of the polar regions in winter, chlorine is released from the reservoirs. On the surfaces of the ice crystals of the clouds, entirely different chemical reactions occur than in the air. Here, the two reservoir substances hydrochloric acid and chlorine nitrate can react with each other, releasing chlorine molecules and nitric acid. The chlorine molecules remain unchanged in the atmospheric air during the winter and do not yet contribute to ozone depletion. The nitric acid is bound in the ice crystals of the clouds, thus causing the NAT clouds described above.

As long as the chlorine is in the form of molecules, it does not lead to ozone depletion. However, as soon as the sun rises in the Arctic spring, the chlorine molecules are dissociated by UV radiation (Lambda <450nm), i.e., split into reactive chlorine atoms. As a result, huge amounts of chlorine atoms are released in a very short time, and an avalanche-like ozone depletion begins, eventually leading to the formation of the Arctic ozone hole. Observing NAT clouds allows for many conclusions about the chemical processes in the upper atmosphere. In the case of the current observation, the stratospheric properties are quite well documented, and there are also several other observations of similar cloud formations from that morning and the previous night from Scandinavia. Therefore, it is not unlikely that it could really have been the first photographic documentation of NAT clouds in our latitudes.

Text: Claudia Hinz