Nacreous clouds

Nacreous clouds resemble thin cirrus or smooth lenticular clouds, but their strong, broad iridescence distinguishes them. The World Meteorological Organization describes the most vivid colours when the Sun is several degrees below the horizon; sunlight still illuminates the high clouds while the lower atmosphere and observer are in twilight. The name refers to mother-of-pearl. Unlike ordinary colourful cloud edges, nacreous displays can cover broad areas and show intense pastel colours.

These clouds form in the stratosphere in very cold conditions, typically near −85 °C, when temperatures fall below the ice frost point. The WMO describes nacreous clouds as ice polar stratospheric clouds, with similarly sized crystals inferred from their characteristic optical effects. The exact particles and composition of polar stratospheric clouds vary: NASA observations also describe clouds involving nitric acid and small amounts of water vapour. Accordingly, nacreous clouds are best treated as the distinctive ice-cloud form of polar stratospheric clouds, not as a synonym for every PSC type.

Nacreous clouds occur mainly at high latitudes during polar winter and are most common in Antarctica, though they have also been observed in the Arctic and in parts of northern Europe and North America. Many are lenticular wave clouds. When stable air passes over mountains, atmospheric gravity waves can lift and cool air in the stratosphere, helping clouds form; severe tropospheric storms may also be associated with their formation. A striking satellite-observed example over Greenland in January 2011 reached an unusual altitude above 30 kilometres, under a rare alignment of mountain waves, the jet stream and the polar vortex. This was an exceptional case, not a typical cloud height.

The brilliant colours arise from diffraction and interference of sunlight interacting with small cloud particles. Their high altitude lets them catch sunlight when the Sun is below the horizon, making them especially visible around dawn and dusk. Some polar stratospheric clouds also affect ozone chemistry: their surfaces can promote conversion of relatively inactive chlorine compounds into ozone-destroying forms, while cloud processes can remove nitrogen compounds that otherwise moderate chlorine activity. This is a property of relevant PSC chemistry, not a claim that every visible nacreous cloud produces a measurable ozone loss.