Extremely rare “rainbow” clouds seen beyond the Arctic Circle: the cause of the phenomenon

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 Extremely rare "rainbow" clouds seen beyond the Arctic Circle: the cause of the phenomenon

Recently, in several places in the Arctic, rare clouds were seen emitting a bright, multi-colored light, similar to the aurora. Live Science writes about this.

Eyewitnesses say that the dark sky suddenly shone with multi-colored light, not caused by the auroras. A riot of color resulted in tiny ice crystals, concentrated in the atmosphere above normal, – polar stratospheric clouds PSC, which form only when the lower stratosphere reaches temperatures below minus 81 degrees Celsius.

Usually clouds do not form at all in the stratosphere due to excessive dryness, but at extremely low temperatures, widely separated water molecules combine into tiny ice crystals that form clouds. This means that PSCs can form much higher than regular clouds, – at an altitude of 15 to 25 kilometers above the ground.

When sunlight passes through these clusters of crystals, it scatters, creating many different wavelengths of light, and therefore colors, which inspired the name “rainbow clouds”. Due to the unusual height, sunlight can hit the crystals and be scattered above the observer, even if the sun is below the horizon. This is when these clouds are at their brightest.

On January 25, extreme cold in the stratosphere led to a rare PSC outbreak above the Arctic Circle, including in Iceland, Norway and Finland.
Jonina Gurun Oskarsdottir made some amazing shots of bright clouds over the top of Mount Jokultindur in Iceland. Photographer Fredrik Broms took his photo series over Kvalloy near Tromsø in Norway.

There are two types of PSC. The former consists of a mixture of ice crystals and nitric acid, which creates less showy colors and may be due to the formation of ozone holes. The second, consisting of pure ice crystals, creates more vibrant colors. Recently formed over the Arctic belong to the second type. They appear in the Arctic no more than two or three times a year, usually during the cold winter months.

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