PSCs are among the highest clouds, forming 15 to 25 km above the ground in a layer known as the stratosphere.
The clouds only form when temperatures in the stratosphere dip below roughly -80C. Typically, the stratosphere is too dry to support cloud formation but, in extremely cold temperatures, sparse water molecules can combine to form ice crystals.
When sunlight hits the crystals, the light gets scattered and splits into different wavelengths, resulting in iridescent colours reminiscent of aurora borealis.
Because PSCs are so high in the atmosphere, sunlight can hit them even after it has dipped beyond the horizon. Often, sky gazers spot PSCs in the hours around sunrise or sunset, when they stand out against dim skies.
The rare outbreak of PSCs in late January coincided with extreme cold temperatures in the polar stratosphere, according to Spaceweather.com. Pearly, shimmering clouds were even spotted as far south as Scotland, the BBC reported. Although PSCs are rarely seen over the UK, a weakening in the polar vortex allowed cold air to sink to lower latitudes.
In Fáskrúðsfjörður, a village on a fjord on Iceland’s east coast, Jónína Guðrún Óskarsdóttir was one of the people who caught a glimpse of the clouds earlier this year.
Usually, the sun can’t be seen from mid-November to late January because of the high mountains in the area, Óskarsdóttir told Cabin Radio by email. When she saw extra light coming through the windows of her house, she knew something was up.
“It’s easy to recognize when the light is different than usual during the darkest days of the winter,” she said.
Óskarsdóttir, who works as the professional director of nursing at a healthcare department on the east coast of Iceland, enjoys nature and photography in her spare time. She has seen nacreous clouds almost yearly during the winter months, she said.
In the daytime, they look like silver in the sky, she said. As sunset approaches, she said the most beautiful colours appear.
“Sometimes they change from one minute to another, which can be mesmerizing.”
A famous painting, an ozone hole
Sightings of PSCs go back to the late 1800s.
Before people took photographs, they depicted the clouds in sketches and paintings. Some people have noted that the sky in Edvard Munch’s famous painting The Scream bears a striking resemblance to a formation of PSCs.
With the advent of satellites, researchers learned that PSCs occur over large areas of both the Arctic and Antarctic. Later, scientists found that PSCs play a role in depleting ozone, which forms a protective layer in the atmosphere that blocks damaging ultraviolet rays from reaching the Earth’s surface.
Although the occurrence of PSCs has roughly held steady over Antarctic regions, some research suggests that PSCs have increased over the Arctic in the past few decades, possibly as a result of climate change.
Óskarsdóttir said the number of her sightings varies from year to year. She hasn’t thought about whether the clouds are increasing.
“I always accept the sight of nacreous clouds as a gift,” she said.