Polar stratospheric clouds (PSCs) are
clouds in the winter polar
stratosphere at altitudes of 15,000–25,000 m (49,000–82,000 ft). They are best observed during
civil twilight, when the Sun is between 1 and 6 degrees below the horizon, as well as in
winter and in more northerly latitudes.[1] One main type of PSC is made up mostly of supercooled droplets of water and nitric acid and is implicated in the formation of
ozone holes.[2] The other main type consists only of ice crystals which are not harmful. This type of PSC is also referred to as nacreous (/ˈneɪkriəs/, from nacre, or
mother of pearl, due to its
iridescence).
Formation
The stratosphere is very dry; unlike the
troposphere, it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their
physical state (super-cooled liquid or ice) and
chemical composition.[3]
Due to their high altitude and the curvature of the surface of the
Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before
dawn or after
dusk.
PSCs form at very low temperatures, below −78 °C (−108 °F). These temperatures can occur in the lower
stratosphere in polar winter. In the
Antarctic, temperatures below −88 °C (−126 °F) frequently cause type II PSCs. Such low temperatures are rarer in the
Arctic. In the
Northern hemisphere, the generation of
lee waves by mountains may locally cool the lower stratosphere and lead to the formation of lenticular (lens-shaped) PSCs.
PSCs are classified into two main types each of which consists of several sub-types
Type I clouds have a generally stratiform appearance resembling cirrostratus or haze.[5] They are sometimes sub-classified according to their
chemical composition which can be measured using
LIDAR. The technique also determines the height and ambient temperature of the cloud.[4] They contain water,
nitric acid and/or
sulfuric acid and are a source of polar
ozone depletion.[6] The effects on
ozone depletion arise because they support chemical reactions that produce active
chlorine which catalyzes
ozone destruction, and also because they remove gaseous
nitric acid, perturbing
nitrogen and chlorine cycles in a way which increases ozone depletion.[7]
Type Ia clouds consist of large, aspherical particles, consisting of nitric acid trihydrate (NAT).[4]
Type Ib clouds contain small, spherical particles (non-depolarising), of a liquid
supercooled ternary solution (STS) of sulfuric acid, nitric acid, and water.[4]
Type Ic clouds consist of
metastable water-rich nitric acid in a solid phase.[8]
Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types [9] and consist of
water ice only.[4]
Only Type II clouds are necessarily nacreous[1] whereas Type I clouds can be iridescent under certain conditions, just as
any other cloud. The
World Meteorological Organization no longer uses the alpha-numeric nomenclature seen in this article, and distinguishes only between super-cooled stratiform acid-water PSCs and cirriform-lenticular water ice nacreous PSCs.[10]