When you look up at the sky, you might see a clear expanse of blue that stretches on endlessly or a stormy mass of gray. Most times, you’ll see clouds – white and fluffy, floating miles above the surface of the earth. However, some extremely rare clouds are colorful, drifting even higher than standard cumulus clouds.
Polar stratospheric clouds, also called “mother-of-pearl clouds” or nacreous clouds, are a type of extremely rare cloud known for the colored light they reflect after sunset and before sunrise. In old English, “nacre” means “mother-of-pearl,” hence the name. Nacreous clouds form as large, thin discs in the lower stratosphere (15-25 km) over polar regions. This is higher than most clouds, which form in the troposphere (10-15 km). The base of nacreous clouds can be 68,500 to 100,000 ft in the air.
The ice particles that compose nacreous clouds are much smaller than those of regular clouds. As light passes through, the light scatters differently, which creates a distinctive colorful appearance. They glow with vivid colors, reminiscent of colors reflected from a thin layer of oil on top of water. Due to the high altitude and curve of the Earth’s surface, clouds are lit up by sunlight from below the horizon and reflect it back to the ground; this is known as cloud iridescence. They shine brightly before dawn and after dusk, when the sun is between exactly one and six degrees below the horizon. They can only be viewed in places with high latitudes, like the Arctic, Scandinavia, and northern Canada.
Polar stratospheric clouds form below -78˚ Celsius, so they can only occur during polar winter. The lack of sunlight in those seasons causes temperatures to drop extremely low. In the Arctic, they are usually made of nitric and sulfuric acid, forming within the stratospheric polar vortex. Because of this, nacreous clouds are associated with very cold and dry weather and have no precipitation.
In the United Kingdom and Ireland, sightings of nacreous clouds have been reported, despite not being nearly as cold as the Arctic. This is due to the movement of the stratospheric polar vortex: it’s much more mobile than the Antarctic vortex, so when it is displaced and hovers over the UK, it’s visible.
Polar stratospheric clouds are categorized as Type I or II based on particle size and formation temperature. Type II clouds, which are the nacreous clouds, are composed of ice crystals and form at temperatures below the ice frost point. Type I clouds are much thinner, from 5 to 8 degrees Celsius above the first point, and are composed of hydrated droplets of nitric and sulfuric acid.
As beautiful as they are, nacreous clouds are destructive and contribute to the breakdown of the ozone layer. The ozone layer is located in the stratosphere and it protects us against UV rays from the sun. Ice crystals inside the clouds give rise to a chemical reaction between the ozone layer (made of O3) and gasses like chlorine and bromine. A single atom of chlorine in the stratosphere can destroy over 100,000 ozone molecules.
Although there have been decades of research into how nacreous clouds affect the climate, it’s still not completely clear because the timing, variability, distribution, etc. are hard to pin down. Scientists are still trying to study these clouds in order to create more accurate ozone depletion models.
Sources:
- https://theconversation.com/explainer-what-are-the-nacreous-clouds-lighting-up-the-winter-skies-54095
- https://www.theguardian.com/environment/2022/feb/10/weatherwatch-how-colourful-nacreous-clouds-are-formed
- https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/other-clouds/nacreous
- https://www.rmets.org/metmatters/nacreous-clouds
- https://www.antarctica.gov.au/about-antarctica/ice-and-atmosphere/atmosphere/clouds-and-radiation/polar-stratospheric-clouds/
- https://www.space.com/what-are-nacreous-clouds-how-do-they-form
