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Psychedelic clouds light up stratosphere

People in Western Finland reported seeing unusual multicoloured clouds around Christmastime. Polar stratospheric clouds only occur in cold enough conditions.

Helmiäispilviä taivaalla.
Nacreous clouds from February, 2017. The phenomenon occurs during the long Finnish winter. Image: Alpo Vuontisjärvi

The sky in Western Finland was lit with Christmas lights on the holiday weekend, as locals reported sightings of multicoloured cloud formations.

The phenomenon is known as polar stratospheric cloud formation (PSC) or nacreous clouds. The mother-of-pearl clouds form at altitudes of some 15-25 km, where water seldom turns to gas. On the western coast low pressure in the lower atmosphere meets the Scandinavian Mountains, pushing the humidity up into the stratosphere where ice crystals take shape and essentially act as prisms.

Helmiäispilviä
PSC is a rare phenomenon. Image: Aki Karjalainen

The psychedelic colour display is caused by the curvature of the Earth, as the nacreous clouds receive sunlight from below the horizon and reflect it to the ground.

Finland's skies have been full of stunning physics moments this year, from aurora borealis to skypunches and meteors burning up in the atmosphere.

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