Polar stratospheric clouds

Arctic PSC

Polar Stratospheric Clouds ( abbr. PSCs of English. : Polar stratospheric clouds ), also called by their appearance pearl clouds known to occur in the stratosphere at altitudes above 20 km along, usually in the range of 22 to 29 km away. PSCs can only form at temperatures below –78 ° C (195 K ). This happens regularly in winter in the polar regions beyond 80 ° north or south latitude . PSCs can be observed much more frequently in the southern hemisphere than in the northern hemisphere (see below for reason).

In the stratosphere, the water vapor content of the air is very low, so that no conventional water clouds can form. Polar stratospheric clouds therefore consist of crystals of sulfuric acid or nitric acid ; At extremely low temperatures, an ice coat can form around these acid crystals. Chemical reactions can take place on the surface of the crystals , which are important for ozone depletion in the stratosphere and the formation of the ozone hole (for the role of PSCs, see there).

Polar stratospheric clouds should not be confused with glowing night clouds . These arise at significantly greater heights of 80 to 85 km in the mesopause .

Appearance and occurrence

The mother-of-pearl coloration of the PSCs is produced by the diffraction and interference of sunlight on the ice crystals and is particularly evident when the sun is already below the horizon .

PSCs occur more frequently in the southern winter than in the northern winter, because an undisturbed polar vortex is a prerequisite for their formation ; this occurs much more frequently in the southern hemisphere. The polar vortex is an area of cold air that forms over the Antarctic continent or the Arctic during the polar night . The land masses of Antarctica are essentially round and there are no major mountain ranges. In the Arctic, on the other hand, the air in the polar vortex has to flow over the mountains of the high northern latitudes, which disturbs the eddy at its edges and warmer air is mixed in. This usually prevents the temperatures in the northern winter from dropping so far that PSCs can develop.

In recent years, however, and particularly in winter 2004/2005, comparatively low temperatures have also been recorded in the Arctic stratosphere. In addition to the Poles, PSCs are being built in Alaska and Siberia as well as in the mountains of Norway and, more rarely, Scotland .

Origin and types

Type II (water) PSC

There are two and three types of PSCs, respectively.

Type I.

Type I polar stratospheric clouds consist mainly of nitric acid and water, they are divided into two sub-types.

Type Ib: solution of sulfuric acid, nitric acid and water

In the stratosphere there is an aerosol layer made up of liquid sulfuric acid droplets (the boy layer ). The sulfur in this layer comes from natural sources: the oceans and volcanic eruptions . Water (H ₂ O) and nitric acid (HNO ₃ ) can deposit on these sulfuric acid droplets at temperatures below –78 ° C (195 K) . Such low temperatures are measured in the stratosphere in the winter half-year at heights between 20 and 30 km. In this process, a formed supercooled ternary solution (engl. Supercooled Solution Ternary STS), which now contains sulfuric acid, nitric acid and water. The polar stratospheric clouds of type Ib consist of these droplets.

Type Ia: crystals of nitric acid trihydrate and water

Due to temperature fluctuations, the nitric acid can escape again from the solution, whereupon crystals of sulfuric acid tetrahydrate ( Sulfuric Acid Tetrahydrate , SAT) remain. When the temperature drops below 190 K, allows the SAT cores condense nitric acid, it forms a shell of nitric acid trihydrate ( Nitric Acid Trihydrate NAT) around the core. Polar stratospheric clouds of type Ia consist of particles built up in this way.

Type II: icy water.

If the temperature drops to –85 ° C (188 K) or below, water can also condense on the SAT / NAT particles that are already present and form an ice coat. Polar stratospheric clouds that are built up from such particles are referred to as type II PSCs. The ice particles with SAT / NAT core are 10 to 1000 µm larger than the particles in PSCs of type Ia or Ib with less than 1 µm.

Trivia

According to Norwegian scientists, mother-of-pearl clouds were the model for the clouds reproduced by the Norwegian painter Edvard Munch in his work entitled The Scream .