photo #5602698

On December 21, 2023 and a couple of days after, a bold display of polar stratospheric clouds, also known as nacreous clouds, formed over parts of Europe. Known for their dramatic iridescent colours, these clouds occur mostly over higher latitudes during the winter months, when particularly cold temperatures in the stratosphere result in ice crystals forming despite the very dry conditions up there.One of the two extreme-altitude formations, nacreous clouds appear at altitudes of around 15 to 25 km (10 to 15 miles) and shine against the darkened sky as they are lit by the Sun when it is just below the horizon. In this particular period, nacreous clouds were visible much further south than usual – as far as southern England, the Netherlands, and even Switzerland and northern Italy, from where they are very rarely spotted. Likely caused by a temporary weakening of the northern stratospheric polar vortex, a circulation of winds around the North Pole up at 50 km (30 miles), which allowed Arctic air in the stratosphere to move further south than normal, the nacreous clouds were the most dramatic over Europe for more than 15 years and featured on national news broadcasts as well as in many spottings by our members.Polar stratospheric clouds spotted over (1) Elgin, Moray, Scotland by Alan Tough (Member 21,958); (2) Mordon, County Durham, England by Sheena Gibson (Member 17,284); (3) Bjørnemyr, Nesodden, Norway by Hanne Ryel; (4) Boston, Lincolnshire, England by Jane Desforges (Member 12,471).