Eyewitnesses say that the dark sky suddenly glowed with a multicolored light, which was not caused by the aurora borealis. The riot of colors was caused by tiny ice crystals floating higher than normal in the atmosphere. These are polar stratospheric clouds (PSCs) that form only when the lower stratosphere reaches temperatures below minus 81 degrees Celsius.

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Normally, clouds do not form at all because the stratosphere is so dry, but at such extremely low temperatures, distant water molecules begin to combine to form tiny ice crystals that form clouds. This means that PSCs can create much higher than normal clouds– at an altitude of 15 to 25 kilometers above the ground.

When? sunlight passes through and diffuses through these crystal clusters, creating light of many different wavelengths and hence the colors that inspired the name “rainbow clouds”. Due to the extreme height, sunlight can hit the crystals and scatter over the viewer even when the sun is below the horizon. This is when the clouds appear brightest.

On January 25, extreme cold in the stratosphere led to a rare outbreak of PSC beyond the Arctic Circle, including Iceland, Norway and Finland. Jonina Gurun Óscarsdóttir took stunning photos of the bright clouds at the top of mountain Jökulsárlón in Iceland:

Rainbow clouds over Iceland / Photo: Jonin Gurun Oskarsdottir

Rainbow clouds over Iceland / Photo: Jonin Gurun Oskarsdottir

Photographer Fredrik Broms took a series of photos over Kvalloja, near Tromsø, Norway.

PSC near Tromso in Norway / Photo: Fredrik Broms

There are two types of PSC:

• The first consists of a mixture of ice crystals and nitric acid, which creates less conspicuous colors and may be related to the formation of ozone holes;
• The latter, consisting of pure ice crystals, creates brighter colors.

Those that have formed over the Arctic recently belong to the second type. They occur no more than two or three times a year in the Arctic, usually during the cold winter months.