Find the warm lid
Look for warmer, drier air above cold, moist low ground. That structure can hold estuary and valley fog beneath the Cadair Idris ridge line.
Eryri / Snowdonia & North Wales
Cadair Idris cloud inversion forecast. Next 24 hours.
See the next 24 hours around Cadair Idris, with a focused southern Eryri map, the strongest dawn window and the valley signals WeatherDuck is watching.
Local outlook
Cadair Idris over the next 24 hours
Very likely
Likely
Possible
Purple dot: Cadair Idris
Latest regional signal
Unlikely
Best window
Tue 22 Sep, 00:00–12:00
Captured 21 Sep, 15:08 BST · model rating, not a literal probability
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Local inversion guide
When can you see a cloud inversion at Cadair Idris?
The best chance comes when colder, moist air settles on low ground beneath warmer, drier air, winds stay light and the cloud top remains below your viewpoint.
Cadair Idris rises between the Mawddach estuary at Dolgellau and the deep trench of Tal-y-llyn. Sea air and valley fog can fill both sides overnight, leaving Penygadair and the Tyrrau Mawr ridge standing above a two sided sea of cloud.
Estuary fog at Dolgellau is a strong sign in settled weather, but watch the layer depth. The Tal-y-llyn side can fill while the estuary stays clear, so agreement between both valleys makes the best mornings.
Before you travel: the map shows the wider pattern across Eryri / Snowdonia & North Wales, not a promise for one summit. Check the latest wind, visibility and mountain forecast for your exact route.
See the forecast data and limitationsCompare both sides of the mountain
The Mawddach estuary and Tal-y-llyn can fill separately. When forecast points on both sides agree, a wraparound cloud sea becomes much more plausible.
Keep cloud below 893 m
Penygadair needs the cloud top below the ridge with clearer air above. A deeper Atlantic layer can put the whole massif inside cloud instead.
Use the calmest window
Calm, cool nights under settled pressure give the fog time to pool. First light along the ridge is usually the best and most photogenic moment.
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Cloud inversion questions for Cadair Idris
Does fog over the Mawddach estuary guarantee an inversion on Cadair Idris?
No. Estuary fog is a useful clue, but the cloud top must stay below the ridge with drier air above. Sea air can also deepen the layer and cover the summit.
Is this a probability for the exact Penygadair summit?
No. The likelihood band is a regional model signal for southern Eryri, supported by nearby captured forecast points. Conditions on the ridge can still differ.
Does a Likely signal mean the Pony Path is safe?
No. The outlook describes cloud structure, not wind, ice, visibility or route conditions. Check the full mountain forecast and local guidance before setting out.
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