Noctilucent clouds

The Sunday Challenge = Nightshot
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Exceptional bright clouds were visible around midnight, last wednesday.

Noctilucent clouds are also called Polar Mesospheric Clouds, PMCs.
They are 80-85 km high (50-53 mile) a few km below the mesopause, the coldest part of the atmosphere.

NLCs are comprised of extremely small ice crystals some 0.1 micron (1/10,000 mm) diameter.

We see them by sunlight scattered by the crystals which are not large enough to show iridescent effects. Their bluish coloration is likely a result of absorption of red light by the stratospheric ozone layer. Occasionally they show reds and golds from the colour of low sunlight illuminating them.

NLC formation requires a combination of very low temperatures, a source of water vapour, and nuclei on which ice can grow.

More info on noctilucent clouds in this link