What is "polar stratospheric cloud"

n. (context meteorology English) A relatively rare type of clouds that occurs high in the stratosphere; polar stratospheric clouds often resemble mother-of-pearl as they shine in vivid colours, known as irisation

Polar stratospheric clouds or PSCs, also known as nacreous clouds (, from nacre, or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitudes of . 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. They are implicated in the formation of ozone holes. 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 destruction.