polar-stratospheric-clouds

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Polar stratospheric clouds (PSCs), also known as nacreous clouds can be seen in the polar countries of the northern and southern hemisphere. They are super pretty but their appearance is actually not a good thing at all: they contribute to destroying our ozone layer which protects life on Earth from harmful ultraviolet light.
Nacreous clouds form in the stratosphere, twice as high as commercial planes fly, where the ozone layer resides. Their ice crystals encourage chemical reactions between human-produced chlorofluorocarbons (CFCs), resulting in the release of chlorine gas, which breaks down ozone.
image credits in captions

Polar stratospheric clouds (PSCs) form in the stratosphere at altitudes of 50,000–80,000 ft. They are classified into Types I (clouds with more diffuse and less bright colors) and II (nacreous or mother-of-pearl clouds), according to their formation temperature and particle size. Unfortunately, Polar stratospheric clouds play a key role in the massive ozone depletion over the Arctic and Antarctic. NASA explains that PSCs “form only at very low temperatures. They help destroy ozone in two ways: they provide a surface which converts benign forms of chlorine into reactive, ozone-destroying forms, and they remove nitrogen compounds that moderate the destructive impact of chlorine. In recent years, the atmosphere above the Arctic has been colder than usual, and polar stratospheric clouds have lasted into the spring. As a result, ozone levels have been decreasing.”

Photo:  NAT GEO

During winter months near the South Pole, the low temperatures cause the pressure to drop to a point where significant condensation occurs (PV= nRT).  This low pressure combined with the wind from Earth’s rotation creates a vortex of spinning air mass with speeds that can exceed 300 km/hr. Polar stratospheric clouds are formed by the condensation of gases in the vortex. Decreases in result in the formation of crystals that contain water sulfuric and nitric acids within these clouds. It is on the surfaces of these clouds that many ozone depleting reactions take place, primarily the formation of chlorine radicals. This is why we have temporary holes in the ozone layer over Antarctica during certain periods of the year.

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These clouds are formed extremely high in the polar stratosphere. Because they are so high they reflect light from below the horizon which results in stunning displays of color. They are also known as nacreous clouds which comes from “nacre” meaning mother-of-pearl and it is easy to see why. These clouds are especially rare because the stratosphere is typically very dry which prevents clouds of any kind from forming.