Nacreous

Polar stratospheric clouds aren't a common sight round our way: indeed, I've only seen them on three occasions in the last eleven years, but they are quite spectacular, even if their cause may include concentrations of nitric and/or sulphuric acid – neither being good for ozone – but other kinds of these clouds contain only water ice.  Either way, it's cold up there, a good 70 or 80 degrees Celcius below zero.

The cold polar winter is partly what enables these clouds to form, so it's not a coincidence that my earlier collections of photographs were taken in December (2012) and February (2016), and two of the three sightings were early in the morning when the sun was barely up.