Polar Stratospheric Clouds

Photo: © by Heino Bardenhagen, Helvesiek / Germany

Photo: © by Jari Piikki, Finnland

Polar Stratospheric Clouds (PSC) form in the stratosphere in altitudes between 20 and 30 km. The formation of PSC requires very low temperatures. That is why they only appear in winter and mainly over Scandinavia, Scotland, Alaska and Antarctica.
In general there are two types of Polar 5tratospheric Clouds:

  1. Type 1: Nitric Acid Trihydrate Clouds (NAT)
    Type-1-PSC-particles are a mixture of water and nitric acid (HNO3) which already form just above freezing point (about -78°C). Subject to the conditions present, they can appear either solid or liquid as aerosols. The microscopic structure of the particles is - seen from the chemical point of view - a connection of each molecule of HNO with 3 molecules of water. This is a so-called thrihydrate with the formula HNO2* 3H2O. Type-1-particles are thus defined chemically as Nitric Acid Trihydrate. (NAT). They are about 1micrometre (one millionth of a metre) small.
    It is supposed that dust in the stratosphere favours the formation of mother-of-pearl clouds because small dust particles are very good sublimation nuclei for water molecules. In Scandinavia, mother-of-pearl clouds can be observed almost every winter. Thanks to the Scandinavian Mountains blocking the westerly winds, Finnish observers can look back over more than 50 times that these clouds appeared within At lower temperatures they can keep on growing and then also include small amounts of chloric acid (HC1) and sulphuric acid (H2SO4). The appearance of NAT-clouds is described as very delicate (similar to NLC) and they are widely spread over a large area.

  2. Type 2: Mother-of-pearl clouds
    Type-2-PSC consist of pure crystals of frozen water. They form at even lower temperatures between -95 C and -90 C and less (188 K at an altitude of 25 km). The particles they consist of are about 10 micrometres small. Due to this the ice crystals are so heavy that these PSC tend to sink down into the troposphere. So the stratosphere which already contains just little amounts of water, becomes even more dehydrated over the poles. PSC of this type, which are also called mother-of-pearl clouds, have a mostly lenticular appearance and appear only over a small area.

Mother-of-pearl clouds

A mostly paste1-coloured iridescence on small ice crystals of lenticular clouds at altitudes between 20 and 30 km is called mother-of-pearl clouds. They are visible best a short time before sunset respectively after sunrise at a distance of 10° to 20° from the sun. But they can also be observed up to 2 hours after sunset, what shows that they are at very high altitudes. They form when a current of air passes over an obstacle, for example a mountain range. This causes an oscillation of the air current and when the atmosphere is steadily piled up, there form steady waves in the lee of the mountains. In these lee waves the air streams up and down for several times. In the areas where it moves up, the air expands and cools down. So vapour can condensate and form clouds. Due to the extremely steady piling of the atmosphere in northern latitudes, the formation of waves reaches up to the uppermost layers of the atmosphere. As temperatures only seldom drop so much, mother-of-peal clouds just form from time to time. Over the arctic and Antarctica, however, according to recent observations, they appear more often over the arctic and Antarctica than it was thought before.
It is supposed that dust in the stratosphere favours the formation of mother-of-pearl clouds because small dust particles are very good sublimation nuclei for water molecules. In Scandinavia, mother-of-pearl clouds can be observed almost every winter. Thanks to the Scandinavian Mountains blocking the westerly winds, Finnish observers can look back over more than 50 times that these clouds appeared within 12 years. There have been speculations about the possibility of mother-of-pearl clouds also over Germany. Theoretically conditions are good at some times in winter also in our latitudes, especially in the north of Germany where the climate of the upper layers of the air is still influenced by the Scandinavian Mountains But observation reports from Germany are very few. From bibliography we know of only one such case. The German "Astronomy News" from 1910 reported that on May 19, 1910, short after Comet Halley bad passed the earth, such clouds had been observed. We should be very grateful for more hints on bibliography about observations of mother-of-pearl clouds over Germany.

Nitric Acid Trihydrate clouds (NAT)

On December 1, 1999, short after sunrise, Heino Bardenhagen watched a sky in Helvesiek, which gave him the impression of a wavy water surface that faintly reflected the sunlight. The pond there appeared upside down. What he observed, was probably another kind of stratospheric clouds. Just at a temperature of -75°C nitric acid (HNO3), which the atmosphere contains in small amounts, can condensate. It forms very thin, fibraciously looking fields of clouds which often extend over thousands of kilometres. As that night similar fields of clouds of that kind had been observed over a large area reaching from the centre of Scandinavia down to northern Germany, he might have observed these so-called Nitric Acid Trihydrate clouds (NAT). The Institutes for Environmental Physics of the universities in Heidelberg and Bremen and the Norwegian Institute for Air Research gave in their ozone bulletins the following information for the corresponding period of time: While during the last winter the stratosphere was relatively warm and only a very low activation of chloride could be measured, the stratosphere cooled down rapidly by the end of 1999, so that in mid December the formation of widespread polar stratospheric clouds became possible. Many ground stations have already observed polar stratospheric clouds. Meteorological temperature analyses from January, 2000, show that the areas with temperatures below 195 K (-78°C) at an altitude of 20 km in the northern hemisphere have never been so large as they are this year.
Those low temperatures are connected with the extreme conditions in the polar areas because the mass of air above the poles is completely isolated from the other global streams of air in winter. As soon as the sun disappears behind the horizon for several months in late autumn, an intensive westerly flow of air forms around the pole which is called the polar vortex. This polar vortex forms an annular stream of air obstructing the exchange of air with the rest of the atmosphere. Only this is the reason why the stratospheric temperatures in this area can drop so much. The polar vortex is especially well-defined in Antarctica because of the great mass of land around the South Pole. The vortex over the Arctic and the processes connected are in general not so well defined.
It bad been predicted that there will be a record loss in ozone over the poles in the following months because according to latest scientific research, stratospheric clouds play an important role in ozone decomposition. Under normal conditions the chloride coming from FCCH set free by man, is bound in the so-called chloride reservoirs. These are chemicals which contain chloride atoms but do not contribute to ozone decomposition. The most important chloride reservoirs are chloric acid (HCI) and chloride nitrate (C10N02). Chloric acid is formed by the reaction of chloride (Cl) with methane (CH4). Chloric nitrate is formed by chloride monoxide (ClO) and nitrogen dioxide (N02). Without these two chemicals, which bind almost all of the chloride in the atmosphere, a lot more of ozone would be decomposed in the atmosphere than really is. So, according to nowadays knowledge, the ozone hole forms because under the special conditions of the polar areas in winter chloride is set free from the reservoirs. The chemical reactions happening on the surfaces of the ice crystal s of the clouds are very different from those in the air. Here the two reservoir substances chloric acid and chloride nitrate can react with each other and set free chloride atoms and nitric acid. In winter, the chloride molecules stay in the air without any modification and still do not contribute to ozone decomposition. The nitric acid is bound in the ice crystals of the clouds and that way forms the NAT clouds described above.
As long as the chloride exists as molecules, there is no ozone decomposition. But as soon as the sun rises in the arctic spring, the chloride molecules are dissociated by the ultraviolet radiation (Lambda less than 450 nm), that means that they are split up into chloride atoms of great reactivity. This sets free large amounts of chloride atoms within a short time and starts an avalanche-like decomposition of ozone which finally leads to the formation of the ozone hole.
So the observation of NAT clouds allows a 1ot of conclusions on the chemical reactions in the upper atmosphere. In the case of the observation present the conditions of the stratosphere are documented rather good and there are also some other observations of similar cloud formations of that morning and the night before from Scandinavia. So it is not improbable that it might have been the first photographic documentation of NAT clouds in our latitudes.

Author: Claudia Hinz

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