Nacreous Clouds

Nacreous clouds develop in high latitudes – 60 to 90 degrees – of the Northern and Southern hemispheres. They are mostly visible within two hours after sunset or before dawn when they blaze unbelievably bright with vivid and slowly shifting iridescent colours. They are filmy sheets slowly curling and uncurling, stretching and contracting in the semi-dark sky.

(Source 1, Source 2)