Rare Polar Stratospheric Clouds outbreak observed over the Arctic
The Arctic skies are lighting up with the iridescent hues of Type II Polar Stratospheric Clouds (PSCs), a rare spectacle created when frigid temperatures transform dispersed water molecules into dazzling icy crystals.
Polar stratospheric clouds seen from Alta, Norway on December 10, 2024. Image credit: Anja, SpaceWeather
An outbreak of polar stratospheric clouds (PSCs), also known as nacreous clouds, has been observed over the Arctic since December 9, approximately one month earlier than the typical season, which in the Arctic usually spans from January to March.
“My favorite clouds appeared this morning,” said photographer Even Nilsen, who captured them from Troms, Norway, on December 10. “It’s the earliest in December I have seen them here in Northern Norway. Fantastic sight in the period of polar night, since we can’t see the sun for the rest of the year.”
“The clouds looked like rainbows… We saw them from 10:00 to 14:00,” said Anja, who took photos of PSC outbreak on the same day from Alta.
PSCs form in the stratosphere when temperatures drop to approximately -78 °C (-108 °F), causing water droplets to coalesce into icy crystals. These crystals refract sunlight, giving the clouds an iridescent glow.
Usually, there are no clouds in the stratosphere. However, PSCs form when temperatures drop significantly, causing dispersed water molecules to coalesce into tiny icy crystals. These clouds form at altitudes of 15 – 25 km (9 – 15 miles) in the lower stratosphere. PSCs consist of similarly sized ice particles, approximately 10 µm across, giving them their characteristic iridescent glow.
PSCs are categorized into two types: Type I and Type II. Type I clouds are composed of nitric acid and water, sometimes containing sulfuric acid, and form at temperatures of around -78 °C (-108 °F). Type II PSCs consist of pure water ice crystals and form when temperatures drop to at least -85 °C (-121 °F).
Type II PSCs are currently being observed over parts of the Arctic including Norway and Sweden. This phenomenon is rare, as it is typically not cold enough for Type II PSCs to form in winter until January. Observing these clouds in mid-December may indicate a season with more active PSC formation.
Type II PSCs are bright and colorful, while Type I PSCs tend to be pale and subdued, making it easier to distinguish between the two. These clouds are often visible as brightly colored wisps during morning and evening twilight.
Type I clouds are further divided into three categories: Type Ia, composed of nitric acid trihydrate (HNO₃·3H₂O); Type Ib, consisting of small spherical droplets of a solution of nitric and sulfuric acid; and Type Ic, which contains small nonspherical particles of metastable nitric acid in the water phase.
While the Type II PSCs, currently being observed, are safe and visually stunning, Type I clouds pose a risk to the stratospheric ozone over the Antarctic and Arctic.
The surfaces of Type I PSCs facilitate the conversion of less harmful man-made chlorine compounds into active free radicals, such as chlorine monoxide (ClO). When sunlight returns to the Antarctic and Arctic in spring, these free radicals trigger chain reactions that destroy ozone molecules.
Another issue with Type I PSCs is that they deplete nitric acid from the stratosphere, which otherwise could react with ClO to form less harmful chlorine compounds.
References:
1 What’s up in space – Spaceweather.com – December 11, 2024
2 Polar Stratospheric Clouds – UCAR – Accessed on December 11, 2024
3 Nacreous and Polar Stratospheric Clouds – Atmospheric Optics – Accessed 11, 2024
Rishav is a skilled researcher specializing in extreme and severe weather reporting. He combines exceptional research capabilities with scientific precision to deliver clear, data-driven articles. Known for uncovering critical information, Rishav ensures his work is accurate, insightful, and impactful. His passion for both science and literature fuels his dedication to producing high-quality news articles. You can reach him at rishav(at)watchers(.)news.
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