Description

Polar stratospheric clouds (PSCs), also known as nacreous clouds from nacre, or mother of pearl, due to their iridescence, are clouds in the winter polar stratosphere.

PSCs are wave clouds. They are often found downwind of mountain ranges, which can induce gravity waves in the lower stratosphere. Their sheet-like forms slowly undulate and stretch as the waves evolve. The clouds can also be associated with very high surface winds, which may indicate the presence of, or induce, winds and waves in the stratosphere.

Formation

PSCs form at temperatures of around -85°C, colder than average lower stratosphere temperatures, and are comprised of ice particles ~10µm across. The clouds must be composed of similar sized crystals to produce the characteristic bright iridescent colours by diffraction and interference.