In late December there was a wonderful display of Nacreous Cloud in the skies over Aberdeenshire. From before sunrise the ethereal colours and effects were evident and continued to varying degrees all day.
Nacreous (mother of pearl) clouds need really specific conditions and are consequently rare phenomena: the cloud needs to be very high at 10-20 miles above the earth's surface, the air at that height needs to be very cold (below -80 degrees Celsius) and a very low sun angle. They're also known as Polar Stratospheric Cloud (PSC) and the particular ice crystals forming the clouds refract the low sun at an angle down to earth. Because of these specific conditions, nacreous clouds are usually only seen in northern polar regions in winter, or when the polar upper atmosphere vortex sinks south, which was what gave rise to this lovely display.
The ice crystals in Nacreous clouds are smaller than those in lower cloud types and can often be composed of atoms of nitric acid and water ice - this combination can combine to release chlorine atoms, so they're not entirely good things!
That said, as the sun began to set in mid-afternoon the colours of the clouds intensified and the shapes changed...it was a captivating sight.
The most intense colours happened well after sunset when the clouds were still being lit by the sun which had left the surface and was shining at just the right angle - simply stunning!