polar-stratospheric-clouds

This speculator photo of Polar Stratospheric Clouds (PSC’s) were captured over NASA’s Radome at McMurdo Station in Antarctica in September 2013.

PSC’s form at an altitude of between 15,000 and 25,000 metres (49,000-82,000 ft) where temperatures of around -85ºC are reached. The clouds are comprised of ice particles around 10 µm across and it is these ice crystals which set the stage for the characteristic bright iridescent colours. The crystals diffract and interfere sunlight when it is low on the horizon; between 1 and 6 degrees.

While these clouds are undoubtedly beautiful, they are also implicated in the formation of ozone holes. Some PSC’s are more exotic in their ingredients and may contain nitric or sulphuric acid. As a result, their surfaces can then act as catalysts which convert chlorine into active free radicals. During the return of spring sunlight these radicals destroy many ozone molecules in a series of chain reactions.

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Image courtesy of Deven Stross

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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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