Nacreous or polar stratospheric clouds form in the winter stratosphere, at altitudes of 15 to 25 km (9 to15.5 miles) and at very low temperatures below −78 °C (−108 °F)
They are seen in winter when the sun is below the horizon, shining brightly well before dawn or after dusk.
This type of PSC is composed of super-cooled ice crystals which are not harmful to the atmosphere.
These were seen from my bedroom window in December last year.
photography group (npub1fcp…w85n)
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