2023-12-23 06:57:00
Contents
The shortest day of the year was gray in the north, but there were colorful clouds in the south. How are these created?
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Legend: Tenero/TI Christine Schelle-Streiff
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Legend: Ascona/TI Barbara Knoth
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Legend: Locarno/TI Marion Schärer
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Legends: Piazzogna/TI Armin Buob
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Legends: Caslano/TI Antonella Beschi
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Legend: Losone/TI Karl Schuler
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Legends: Miglieglia/TI Reto Busenhart
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Legend: Ascona/TI Rosemarie Luebkert-Kamermans
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Legend: Locarno/TI Peter Bruegger
When clouds or soap bubbles glow colorfully, it is called iridescence. Such iridescent effects are most often seen in cirrocumulus clouds or lenticular clouds in the lee of mountains – i.e. on the side facing away from the wind – such as the Alps. In the images from Friday, December 22nd, cirrocumulus clouds are most likely not causing the spectacle. You can see so-called “polar stratospheric clouds”or short PSCs from English for polar stratospheric clouds.
Why PSCs: The clouds were still visible long after sunset, while “normal” clouds had already lain in the dark in the Earth’s shadow for a long time. PSCs look very similar, but are not found at an altitude of around 10 km like cirrus clouds, but at an altitude of 22 to 29 km. In the stratosphere, i.e. above the weather layer, there is very little water vapor. Polar stratospheric clouds therefore consist of crystals of sulfuric acid or nitric acid. They only develop at temperatures around -80 degrees. They are therefore mainly found in the polar regions during the winter.
PSCs attack the ozone
But PSCs are not only beautiful to look at, they are also destructive. Special chemical reactions take place on the surface of the cloud crystals – for example with chlorine. They deplete the ozone. On the one hand, this led to the ozone hole and, as a result, to the ban on harmful chlorofluorocarbons as coolants.
Very rare
PSCs over Switzerland are very rare. They are slightly more common in northern Norway, Alaska and Iceland. On average, you can admire them there on around 10 days per year.
No glowing night clouds
Something different again are noctilucent clouds. These are bright, but not necessarily colorful, clouds that are visible after sunset. They arise at an altitude of over 80 km.
How does iridescence occur?
According to Duden, “iridescence” means “to shimmer or shine in rainbow colors”. Iridescent clouds are created by the diffraction and interference of sunlight within the cloud.
Light can be illustrated as rays of light or as waves. The wavelength of red light is longer than the wavelength of blue light. And white sunlight consists of all colors.
Now imagine two waves in the water moving towards each other and eventually meeting. If the wave crests meet, they add up; if a wave trough meets a wave crest, the waves can cancel each other out. And that’s exactly what happens in the colored clouds, simply not with water waves, but with light waves.
Iridescent clouds contain very small ice particles or water droplets that are only 0.01 to 0.02 mm in size. These particles bend sunlight, thus redirecting the light waves. This causes the light waves to overlap, amplify or cancel each other out. It can happen that short waves cancel each other out and you only see long-wave red light, or that the yellow wavelength increases and you see a yellow zone in the cloud.
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