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.
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.