Early December – Nacreous or polar stratospheric clouds: a rare winter sky phenomenon

Nacreous clouds: a rare winter sky phenomenon

As we edge into December, we’re entering a magical window of time when nacreous clouds—those rare, otherworldly displays—might make an appearance once again. These clouds, also known as polar stratospheric clouds (PSCs), are a stunning meteorological phenomenon that typically form during the coldest part of winter. Last December, the UK was treated to an incredible spectacle, with vibrant displays lighting up the sky. I saw them while flying to Iceland and again on one of the days there. It was a moment I’d always dreamt of, yet I couldn’t believe it was actually happening. To see them so vividly with my own eyes was surreal.

What are nacreous clouds?

Nacreous clouds form high in the stratosphere at altitudes of 9 to 16 miles (15 to 25 km), owing their iridescent glow. Think delicate pinks, greens, blues, and violets—to the way sunlight diffracts through tiny ice crystals in the clouds. These breathtaking colours appear most vividly just before sunrise or after sunset when the sun sits just below the horizon, casting its light onto these high-altitude formations.

How nacreous clouds form: extreme cold and altitude

These clouds form under extreme cold, with temperatures dropping below -78°C (-108°F), which is why they’re most often seen in the polar regions during winter. The freezing stratospheric conditions allow supercooled water, nitric acid, and sulphuric acid to condense, creating the tiny cloud particles that paint the sky with their pearl-like brilliance.

How the UK got to witness nacreous clouds

Last winter’s spectacular nacreous cloud display over the UK happened because of an unusual shift in the polar vortex. Typically confined to the polar regions, the polar vortex moved south. Bringing the necessary freezing temperatures in the higher altitudes of the atmosphere overhead in the UK. This rare shift allowed the unique conditions that give birth to these clouds to form in British skies. The result was a breathtaking. Unexpected show that had skywatchers in awe, as the vivid colours lit up the winter sky.

Types of nacreous clouds: what makes them so special

There are two types of nacreous clouds, each with distinct characteristics:

Type I PSCs:

These clouds are made from a mix of ice, nitric acid, and sulfuric acid. While less visually striking, they play a crucial role in chemical reactions that deplete the ozone layer, highlighting their environmental significance.

Type II PSCs:

The real showstoppers. Composed almost entirely of water ice, these clouds are responsible for the breathtaking iridescent colours that captivate sky watchers. They’re much rarer and an awe-inspiring sight to behold.

Why are nacreous clouds important for the environment?

Despite their ethereal beauty, nacreous clouds are intricately tied to a sobering environmental impact. The surfaces of these clouds act as catalysts for ozone-depleting reactions. Converting inactive chlorine compounds into reactive forms that break down ozone molecules. This process plays a key role in the depletion of the ozone layer over the poles. Though chlorofluorocarbons (CFCs) have been banned in the UK since 2010 and globally since 2010, the gases can take 50 years to break down. A stark reminder of the legacy of some of the disastrous decisions about chemicals and substances that humans have made.

Where and when to see nacreous clouds

Nacreous clouds are most frequently observed in the polar regions, particularly in Antarctica and the Arctic. However, when conditions are just right—such as during the particularly cold winters in the UK when the polar vortex shifts south—you might be lucky enough to catch a glimpse of these mesmerising formations. They’re best seen in the winter months when the stratosphere reaches the frigid temperatures necessary for their formation.

A climate connection: what nacreous clouds tell us about the atmosphere

Beyond their captivating appearance, nacreous clouds offer insight into the state of our atmosphere. Their connection to ozone depletion, the structure of their small ice crystals act as a nuclei for chlorine and bromine to break down on. Their increased visibility in colder-than-usual climates signal larger changes in the environment. As the climate shifts, these clouds not only create a visual spectacle but also serve as a reminder of the vulnerabilities in our planet’s atmospheric systems.

Look up this December: the magic of nacreous clouds

As we approach the coldest part of winter, keep your eyes to the sky this December. The vivid displays we witnessed last year across the UK could return. Offering a fleeting but spectacular view of one of nature’s rarest phenomena. Nacreous clouds—ethereal, fragile, and stunning—remind us of both the beauty and fragility of our world. And in moments like these, the sky holds a mirror to the shifting stories of our planet.

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