vivid and lustrous, wafting iridescent waves of colour wash across this skyscape from Kulpizhavi, Finland. Known as Nakhreus clouds or mother of pearl clouds, they are rare. But their unforgettable appearance was captured looking south at 69 degrees north latitude that sunset on January 24. A type of polar stratospheric cloud, they form when unusually cold temperatures in the usually cloudless lower stratosphere form ice crystals. Still sunlit at altitudes of around 15 to 25 kilometres, the clouds can defract sunlight even after sunset and just before the dawn.