Rare, mysterious and beautiful polar stratospheric clouds have dazzled sky-gazers with their rainbow-coloured hues this winter. Thanks to EarthCARE’s newly released Level 1 data, we can observe them in great detail from space.

The European Space Agency’s Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite launched in May 2024 to observe clouds and aerosols and the effect they have on Earth’s radiation balance. Its data were released to the public in January 2025, and are already showing impressive results.

January brought with it some very cold weather due to changes in the Arctic polar vortex, a band of wind high up in the stratosphere that circulates at very high speeds of around 250 km/h. Changes in the polar vortex can result in cold Arctic air moving further south, this time trapping Northern Europe in an icy grip.

In conditions of extremely cold temperatures, below -78°C, polar stratospheric clouds (PSCs) can form at altitudes of around 20-30 km. These rare clouds are iridescent, appearing rainbow-like in the sky. They are often known as acid clouds because they are commonly formed from a mixture of water and nitric acid.

The distribution, occurrence and profiles of PSCs have been somewhat of a mystery, therefore EarthCARE’s latest observations have been of great interest to cloud researchers.

EarthCARE shines a light on polar stratospheric clouds

PSCs were observed by EarthCARE on 13 January 2025, as it travelled over northern Europe, from Ukraine to northern Greenland. A long band of these clouds stretched approximately 3,000 km from Latvia, over Finland, Sweden, and Norway, to Greenland at heights of between 20 and 30 km.

In the Arctic, PSCs mostly consist of tiny acid droplets, known as Type I, but EarthCARE’s Atmospheric Lidar (ATLID), which uses a laser to detect clouds and aerosols, also detected those consisting of ice crystals, known as Type II.

Type II PSCs are very rare in the Arctic due to the very low temperatures required for their formation, and much more commonly form in Antarctica where temperatures frequently dip below -88°C.

According to the European Centre for Medium-Range Weather Forecasts (ECMWF) weather model, the temperatures in these clouds reached as low as -97.3°C.

“With ATLID it is possible to clearly see waves of forming ice PSCs, which was also predicted in the forecast maps,” says Ulla Wandinger, Aerosol Lead for the EarthCARE Data, Innovation and Science Cluster (DISC). “The temperature drops below -90°C in the updrafts of the waves, which is where ice crystals form.

“The ice crystals cause the high signals in ATLID’s cross-polar backscatter channel because these irregularly shaped solid particles depolarise the backscattered light. In contrast, the more common Type I PSCs, consisting of spherical droplets, do not cause depolarisation.”

EarthCARE can provide insights into phenomena such as these. For example, these rare Arctic PSCs can be compared to the more common Antarctic PSCs seen during the southern-hemispheric winter, which were also detected by ATLID during the satellite’s commissioning phase in August 2024.

“EarthCARE’s ability to view up to 40 km shows the full extent of these polar stratospheric clouds and this range is one of the many benefits of the EarthCARE mission,” says Gerd-Jan van Zadelhoff, EarthCARE DISC project manager. “As EarthCARE was launched in May 2024, just before the start of winter in the Southern Hemisphere, it means that we are already able to compare the signals from the Arctic cloud formations with those seen in the Antarctic exactly five months earlier.”