Nacreous Clouds Explained: Polar Stratospheric Clouds, Types I and II

As highly rare polar stratospheric clouds, nacreous clouds are bright, colourful clouds that form in the winter stratosphere at altitudes of about 15 to 25 kilometres. Nacreous clouds require stratospheric temperatures below the ice frost point, typically near –85°C (not –78°C), and are composed primarily of ice crystals; nitric acid particles are characteristic of Type I polar stratospheric clouds, not nacreous (Type II) clouds. They are seen mainly over Antarctica and, far more rarely, from places such as Hobart in Tasmania, with occasional sightings from southern Tasmania and the Hobart region when conditions are right.

Key takeaways

• Nacreous clouds are rare polar stratospheric clouds that form well above ordinary weather.
• They need stratospheric temperatures below -78°C before they can form.
• Their colour comes from atmospheric optics, especially diffraction and iridescence.
• Type I PSCs contain nitric acid trihydrate, while Type II PSCs are made of water ice.
• These clouds help chlorine activation in the Antarctic stratosphere, which contributes to ozone depletion and the Antarctic ozone hole.

The formation of polar stratospheric clouds

The atmosphere is usually described in layers, and most everyday weather happens in the troposphere. Polar stratospheric clouds form much higher, in the stratosphere, where temperatures can fall sharply during the polar winter. The Bureau of Meteorology describes cloud iridescence as an optical effect seen when sunlight diffracts off water or ice crystals in the sky, and the same principles help explain why nacreous clouds can look so vivid.

Over Antarctica, the polar vortex traps very cold air above the continent. That setting is most important for PSC formation. Surface weather systems, including extreme-low-pressure systems, are not the main cause of nacreous clouds because these clouds form high in the stratosphere rather than near the ground. At the Antarctic Research Stations, scientists monitor this cold season closely because it is when PSCs are most likely to appear.

What are nacreous clouds and how do they form?

Nacreous clouds are a type of polar stratospheric cloud that form in the winter stratosphere at heights of about 15 to 25 kilometres. They usually need temperatures below -78°C. At that point, water vapour and other compounds can freeze into tiny particles, creating the conditions for mother-of-pearl clouds and the colours associated with them. These clouds are most often seen over Antarctica and only rarely from Hobart.

Why are nacreous clouds so colourful?

The bright colours come from atmospheric optics. As sunlight passes through tiny, similarly sized ice crystals or droplets, it is split by optical diffraction and interference. That effect creates the soft pastel shades seen in nacreous clouds and other iridescent clouds. Because the clouds sit high in the stratosphere, they can still catch sunlight after the Sun has dropped below the horizon for observers on the ground, which makes the colours stand out.

Type I vs Type II nacreous clouds

Meteorologists separate polar stratospheric clouds into two main groups based on their composition and the temperature at which they form. These are Type I PSCs and Type II PSCs. The Bureau of Meteorology uses this kind of classification when describing polar cloud processes, and it helps explain why some clouds are more strongly linked to ozone chemistry than others.

Type I PSCs are mainly made up of nitric acid trihydrate and other particles containing nitric and sulfuric acids. They form at around -78°C. They are important in stratospheric chemistry, but they do not always show the bright mother-of-pearl colours that people expect from classic nacreous clouds.

Type II PSCs are made of pure water ice. They form at even lower temperatures, typically below about -83°C. These clouds are the ones most closely linked with the most striking nacreous displays, because their ice crystals are well suited to produce the strong iridescence seen in the sky.

How nacreous clouds contribute to ozone depletion

Nacreous clouds are more than a visual spectacle. They play a direct part in the chemistry that drives ozone loss over Antarctica. On the surface of PSC particles, compounds that contain chlorine can be converted into forms that become highly reactive once sunlight returns in spring.

This process is known as chlorine activation. After sunlight reaches the polar stratosphere, the reactive chlorine helps break down ozone, which contributes to ozone depletion and the growth of the Antarctic ozone hole. The link between PSCs and ozone loss is one reason Antarctic Research Stations continue to track these clouds so closely.

Why are nacreous clouds important for the ozone hole?

They matter because they provide the cold surface needed for the chemistry that turns inactive chlorine into ozone-destroying compounds. Once spring sunlight arrives, that chemistry speeds up. The result is faster ozone loss over Antarctica, which is part of the wider Antarctic ozone hole problem.

Southern Hemisphere sightings and Australian viewing chances

Most nacreous clouds are seen over Antarctica, where the air is cold enough for long periods. In the Southern Hemisphere, sightings are rare outside the Antarctic region, but observers in Hobart and southern Tasmania may occasionally see them when the polar vortex extends far enough north and the sky is clear.

Are nacreous clouds rare in Australia?

Yes. They are very rare in Australia because the stratosphere rarely becomes cold enough over the country. When they do appear, they are more likely to be reported from Hobart, Tasmania, or coastal southern locations that have a clear view of the southern sky. Even then, sightings remain unusual.

Weather note
Mountain wave activity can help trigger local cooling in the upper atmosphere, but nacreous clouds still need the broader Antarctic stratosphere to be extremely cold. That is why the best observations usually come from Antarctic Research Stations rather than from ordinary surface weather events in Hobart or elsewhere in Tasmania.

The role of Mawson and Davis stations in PSC research

Australia’s Antarctic Research Stations play an important part in monitoring PSCs. At Davis Station, Mawson Station and Casey Station, researchers use radiosondes, ozonesondes and other instruments to measure stratospheric temperature, cloud height and ozone levels during the polar winter.

These observations help scientists track when temperatures drop below -78°C, when Type I PSCs and Type II PSCs form, and how chlorine activation changes as spring sunlight returns. The Bureau of Meteorology also contributes to public understanding of cloud optics and weather observation across Australia, including in Hobart and southern Tasmania.