56° N. “That is what it is! Drinkable light!”

This is why I was standing in a field at 2am this morning with my jammies on under my joggies. I would have stayed longer but nocturnal rustlings in the undergrowth were giving me the heebies.

Noctilucent Clouds are also called the fancier name Polar Mesospheric Clouds which describes where they originate ; 80-85 km high (50-53 mile) a few km below the mesopause, the coldest part of the atmosphere.

NLC formation requires a combination of very low temperatures (-123°C!), a source of water vapour, and nuclei on which ice can grow.

Looks like mankind is causing an increase in their frequency and presence at lower latitudes - the summer mesopause is getting colder, possibly because of the cooling effect of increased atmospheric carbon dioxide concentrations...