Imaged by Rob Stewart (photography) 18th February and 51 minutes after
sunset from Mull loch na keal, Western Scotland
The UK, The Netherlands, Scandinavia and parts of Germany experienced a series of unusually bright and intensely colourful sunset and dawn twilights over the last few days. They are sometimes caused by volcanic dust ejected into the stratosphere but this is unlikely to be the reason for the recent ones. Europe's cold anticyclonic weather could also have caused a build up of tropospheric haze and aerosol with clear air above that enhanced the twilight.
Another and more likely explanation is the presence of very high (15-25 km) and thin 'Type1' polar stratospheric clouds, PSCs. The exceptionally low temperatures needed to form them have existed over the area of Europe where the twilights have been sighted for the last few days. Type1 PSCs composed of nitric acid compounds are less colourful and bright than the better known Type2 or nacreous clouds made of ice crystals but here they have scattered high altitude sunlight to give us some truly memorable twilight skies.
Image ©Rob Stewart , shown with permission.