January 2024

Nacreous: the Colourful Cloud

Most of Earth’s clouds form in the lowest part of our atmosphere, known as the troposphere, which extends up to around 12 km (7.5 miles). But a couple of cloud types exist at a far more elevated plane, and one of these extreme-altitude formations, the nacreous cloud, is the most colourful to grace our skies. 

Known more officially as a Type II polar stratospheric cloud, this rarely seen ice-crystal cloud develops way up in the stratosphere, the layer above the troposphere. It isn’t an easy place for clouds to form because it is very dry up there. You see, the boundary between the troposphere and stratosphere, known as the tropopause, acts as an invisible lid holding down any moisture lifting from the planet’s surface below. Since the air is so dry up above this atmospheric lid, clouds only form in the stratosphere when the temperatures are low. Very low. 

For nacreous clouds to form, air temperatures in the stratosphere need to drop to at least –85°C (–120°F). Only then will the rarefied water molecules start to stick together into ice crystals. Low temperatures like this tend to happen only in the winter and over higher latitudes. Nacreous are therefore wintertime clouds spotted mostly from locations like northern Canada, Antarctica, and northern Scandinavia. But in late December 2023, a massive display of these elusive clouds occurred in the Northern Hemisphere over larger regions than normal. Spotted here by Susan Will from a flight into Aberdeen, Scotland, these nacreous lit up the skies as far south as Switzerland and Italy with characteristically striking iridescent hues.

If nacreous are Type II polar stratospheric clouds, what about Type I? These are a different sort of stratospheric cloud, made not of ice crystals but droplets of nitric and sulphuric acids. They have the harmful effect of depleting the all-important ozone layer, and they show only subtle, flat colourations, not the glorious spectral hues produced by the ice-crystal nacreous clouds.

The mother-of-pearl colours appear because nacreous clouds have ice crystals of a uniformly tiny size – just 0.002 mm across. Their small and consistent size means the crystals scatter, or diffract, the sunlight passing around them very effectively, separating out its visible wavelengths to appear as wavy bands of iridescent colour. Normal clouds in the troposphere below can produce their own form of cloud iridescence, but nacreous display far more pronounced and dramatic light effects, their shimmering tints most vivid before sunrise or after sunset when the Sun’s just below the horizon.

Nacreous clouds with Stratocumulus below, spotted on a flight into Aberdeen, Scotland by Susan Will on December 21, 2023 during the most dramatic display of these clouds over Europe since 2006. View this image in the photo gallery.

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