It's the start of Cloud Inversion or Temperature Inversion season.
Cloud inversions occur in still air when temperature increases with air height, and night mist and fog can get trapped in the cooler air lower down. This can cause some stunning cloud inversions, MWIS often report whether an inversion is likely, set your alarm clock and get ready to head up a hill early. Sometimes cloud inversions especially the deeper we get into Autumn and into winter last all day long, so if you are not an early riser you can still catch one during the day or even at sunset as the clouds roll in.
If you are heading out early or late, please do remember to take a head torch, and know your route.