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 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][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 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.[9]
Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types [10] and consist of water ice only.[4]
A stratiform type I PSC (white cloud above the orange tropospheric clouds), showing fine horizontal structures in the veil
