How autumnal high pressure brings on the dark doldrums

In summer, a high-pressure system over the UK usually means fine weather. But in the colder months it can result in anticyclonic gloom, characterised by grey skies, mist, fog and drizzle.A settled high-pressure system brings stable conditions and impedes the usual vertical movement of air. This is a recipe for sunshine when humidity is low, but can act like a lid, trapping atmospheric moisture near the surface.The moisture condenses as low stratus or stratocumulus clouds, and sometimes as fog or mist. Wind normally disperses these and in summer the sun is strong enough to burn them off. But there is little wind when high pressure dominates, and in winter the shorter duration and low angle of the sunlight make it less effective.The weather may be mild and even warm during anticyclonic gloom, but dull conditions persist day after day, with frequent drizzle and occasional mist or fog.In northern Europe in the renewable energy sector, this situation is called Dunkelflaute, German for “dark doldrums” because it brings a reduction in both solar and wind power. Spells of such weather pose challenges for the smooth transition to renewable energy, highlighting the need for pumped storage or other backup energy sources until the high pressure moves on and the weather changes.