Will there be a cloud inversion at sunrise?

A cloud inversion is a sea of cloud seen from above. It forms when a layer of warmer air sits on top of colder, damp air near the ground. The warm layer acts as a lid, trapping fog or low cloud in the valleys, so anyone on a hill above the lid looks down on a flat, white cloud sea in clear sunshine.

Not quite. A temperature inversion is the warm layer itself: air that gets warmer with height instead of colder. A cloud inversion is what you see when there is also enough moisture underneath for fog or cloud to form, with clear air above. A temperature inversion without moisture just gives a clear, frosty valley.

The best set-up is high pressure, light winds and a clear, long night after a damp day. The ground cools quickly under clear skies, cold air drains into the valleys and condenses into fog, and sinking air in the high pressure builds a warm lid above it. Strong winds, rain or a lot of high cloud usually stop an inversion forming.

Around sunrise. Valley fog is usually deepest at dawn and burns off within a few hours as the sun warms the ground, often lifting into a layer of low cloud first. Under a strong winter high, a cloud inversion can last all day or even several days.

Autumn and winter are best, roughly October to February in the UK. Long nights give the ground more time to cool and the low winter sun struggles to clear the fog. Radiation fog peaks in autumn, while long-lasting inversions under high pressure are most common in midwinter.

You need to be above the top of the cloud. Shallow valley fog can be just 50 to 200 metres deep, so a small hill is often enough. Inversions under high pressure usually have cloud tops between 500 and 1,500 metres. This checker estimates the cloud top for each morning and tells you how high to aim; add 100 metres or so for safety, because cloud tops ripple and rise as the morning warms.

Places where hills rise above deep valleys or lakes: the Lake District (Latrigg, Cat Bells, Helvellyn), the Peak District (Mam Tor and Win Hill above the Hope Valley), Eryri (Moel Siabod, Yr Wyddfa), the Scottish Highlands (Ben Nevis, Conic Hill above Loch Lomond), Bannau Brycheiniog (the Sugar Loaf and Pen y Fan above the Usk valley) and the Yorkshire Dales.

It can be, with planning. Use public paths and open access land, follow local laws and access codes, and pick a route you know or have checked in daylight. Check the mountain forecast, tell someone your plans, take a headtorch, map and warm layers, keep away from cliff edges in fog, and turn back if conditions change. In an emergency in the UK, call 999 and ask for the Police, then Mountain Rescue.

A Brocken spectre is your own shadow cast onto cloud or fog below you, often surrounded by a rainbow-coloured ring called a glory. You need the sun low behind you and cloud in front of you, which makes a cloud inversion at sunrise one of the best times to see one.

It is a guide based on weather model data, not a guarantee. Fog is one of the hardest things to forecast, and weather models smooth out narrow valleys, so the forecast is most reliable for broad, settled set-ups the evening before. Always check a local mountain forecast and conditions before heading out in the dark.