First documented appearance of a polar stratospheric cloud over Switzerland and Italy, seen from Brissago, Ticino, Switzerland on December 22, 2023
Apolar stratospheric cloud (PSC) is acloud that forms in the winter polarstratosphere at altitudes from 15,000 to 25,000 m (49,000 to 82,000 ft). They are best observed duringcivil twilight, when theSun is between1° and 6° below thehorizon, as well as inwinter and in more northerly latitudes.[1] One main type of PSC is composed of mostlysupercooled droplets of water andnitric acid and is implicated in the formation ofozone holes.[2] The other main type consists only ofice crystals, which are not harmful. This type of PSC is also callednacreous (/ˈneɪkriəs/; fromnacre, ormother of pearl), due to itsiridescence.
The stratosphere is very dry; unlike thetroposphere, 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 theirphysical state (super-cooled liquid or ice) andchemical composition.[3]
Due to their high altitude and the curvature of the surface of theEarth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well beforedawn or afterdusk.
PSCs form at very low temperatures, below −78 °C (−108 °F). These temperatures can occur in the lowerstratosphere in polar winter. In theAntarctic, temperatures below −88 °C (−126 °F) frequently cause type II PSCs. Such low temperatures are rarer in theArctic. In theNorthern Hemisphere, the generation oflee waves by mountains may locally cool the lower stratosphere and lead to the formation of lenticular (lens-shaped) PSCs.
A lenticular type II (water) PSC showing iridescence
PSCs are classified into two main types, each of which consists of several subtypes.
Type I clouds have a generally stratiform appearance resembling cirrostratus or haze.[5] They are sometimes sub-classified according to theirchemical composition which can be measured usingLIDAR. The technique also determines the height and ambient temperature of the cloud.[4] They contain water,nitric acid and/orsulfuric acid and are a source of polarozone depletion.[6] The effects onozone depletion arise because they support chemical reactions that produce activechlorine which catalyzesozone destruction, and also because they remove gaseousnitric acid, perturbingnitrogen and chlorine cycles in a way which increases ozone depletion.[7][8]
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 liquidsupercooled ternary solution (STS) of sulfuric acid, nitric acid, and water.[4]
Type Ic clouds consist ofmetastable water-rich nitric acid in a solid phase.[9]
Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types[10] and consist ofwater ice only.[4]
A stratiform type I PSC (white cloud above the orange tropospheric clouds), showing fine horizontal structures in the veil
Only Type II clouds are necessarily nacreous[1] whereas Type I clouds can be iridescent under certain conditions, just asany other cloud. TheWorld 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.[11]
