photo #5746794

Nacreous, or polar stratospheric, clouds occur mostly at high latitudes (near the poles) during the winter months, when temperatures up in the stratosphere are at their lowest. This is because nacreous clouds form at extreme altitudes of around 10 to 15 miles (15 to 25 km). Being so high, they can catch the light when the Sun is well below the horizon and all the lower sky is in shadow. A popular time to spot nacreous clouds is just after sunset, when they shine against a darkening sky with iridescent colours.The colours are the result of light diffracting around these clouds’ very small and uniformly sized ice crystals. It is rare to spot a nacreous cloud in the middle of the night, when the Sun can’t shine onto it directly. But that is when Dan Raymond spotted this nacreous cloud, lit with pearlescent hues by the Moon over Richmond, North Yorkshire, England. While iridescent colours can appear in normal lower clouds, this one can be identified as the extreme-altitude nacreous formation by its thin, blurry-edged appearance and the intensity of the iridescence. It was an unusually southerly occurrence of the formation since it normally forms at much higher latitudes. It was part of a particularly intense display of nacreous clouds over Europe in late December 2023. Nacreous clouds are the sky equivalent of bioluminescent deep-sea comb jellies.