Moel Siabod cloud inversion forecast
872 m · Snowdonia
Moel Siabod is the solitary mountain above Capel Curig, climbed by the rocky Daear Ddu ridge past Llyn y Foel or a simple path from Plas y Brenin.
A cloud inversion is when you stand on a summit in sunshine above a sea of cloud that fills the valleys. This morning's chance on Moel Siabod is 2% (Low), confidence high. Too windy at summit height for fog to settle; air aloft is mixing, not layered.
Cloud inversion forecast for Moel Siabod, next 7 days
| Morning | Chance | Rating | Best time |
|---|---|---|---|
| Today | 24% | Low | 08:00 |
| Monday 19 Oct | 0% | Low | 04:00 |
| Tuesday 20 Oct | 3% | Low | 07:00 |
| Wednesday 21 Oct | 3% | Low | 04:00 |
| Thursday 22 Oct | 1% | Low | 06:00 |
Ratings: Excellent 70% or more, Good 50%, Marginal 30%, Low under 30%.
What is driving this morning's chance
- Wind at summit height: 21 mph (holding it back)
- Stability aloft: -7.6°C/km between 822m and 1503m (holding it back)
- Valley moisture: Dew point spread 1.5°C, low cloud 0% (neither helps nor hinders)
- Summit above the cloud: Fog top ~565m, summit 872m (helps)
- Dry air above: Humidity 83% at 1503m (neither helps nor hinders)
- Clear night: Cloud 91% overnight (holding it back)
- Overnight cooling: Valley 3.6°C colder by dawn (neither helps nor hinders)
- High pressure: 1022 hPa (sea level) (neither helps nor hinders)
- Calm valley: 4 mph in the valley (helps)
- Night length: 13.4 hours of darkness (helps)
How the Moel Siabod cloud inversion chance is worked out
Weather models are too coarse to see fog lying in a single valley, so Peakify scores the setup a mountain forecaster looks for. It compares the forecast for the summit (872 m) with the lowest ground within 6 km (215 m here), and scores each morning at its best hour before and around dawn.
Two things act as gates: light wind at summit height, and a fog top that stays below the summit. If either fails, the chance stays low whatever else is happening. The rest builds the score: a stable, warmer layer of air above the valleys, moist valley air close to its dew point, dry air above the summit, a clear night, high pressure, a good drop in valley temperature overnight, a calm valley floor and a long night.
On Moel Siabod the summit stands about 657 m above that valley floor, so a fog layer has to stay shallower than that for you to be above it.
How a cloud inversion forms on Moel Siabod
After a damp day the wind has dropped and the sky has cleared. The air in the valleys still holds plenty of moisture.
What if…
A sea of cloud is likely: calm, clear and damp.
Read it step by step
- A calm, clear evening. After a damp day the wind has dropped and the sky has cleared. The air in the valleys still holds plenty of moisture.
- Heat escapes to the sky. With no cloud overhead to act as a blanket, the ground radiates its warmth straight out to space, and the air touching it starts to chill.
- Cold air drains downhill. Chilled air is denser than the air around it, so it slides down the slopes like water and gathers on the valley floor.
- The inversion forms. A pool of cold air now fills the valleys with milder air above it. Normally it gets colder as you climb; tonight it gets warmer. That flip is the temperature inversion.
- Mist over the water. Lakes and wet ground give the cold air the last bit of moisture it needs, so wisps of mist form there first. If you are going up, this is when to set off, by head torch.
- Fog fills the valleys. The trapped air keeps cooling until it reaches its dew point. Its moisture condenses into tiny droplets, and the cold pool turns to fog. Climbing through it, you see nothing but grey.
- A sea of cloud. The warm layer above works like a lid, so the fog cannot rise through it. Summits higher than the lid break out into clear air and sunshine.
- The best hour on top. Low sun lights the cloud tops gold and the hills cast long shadows across them. Turn your back to the sun: your own shadow may land on the cloud inside a rainbow ring, a Brocken spectre.
- The sun gets to work. Sunlight warms the slopes and the ground beneath the fog. Rising thermals stir the air and start to break the lid.
- The inversion lifts. The cloud rises, thins and breaks apart, usually by late morning. Then it is an ordinary day again, until the next calm, clear night.
When cloud inversions happen on Moel Siabod
Inversions need a calm, clear night under high pressure, when cold air drains into the valleys and the moisture in it condenses into fog. That happens most in autumn and winter, when nights are long enough for the valleys to cool, and in settled spells after rain has left the ground wet. They are a morning event: be on the top at or before sunrise, as the sun usually starts to burn the fog off within a few hours.
A high inversion chance is not a forecast of good walking. The same calm conditions can bring frost and ice on the paths, and if the fog is deeper than forecast you will be climbing in it all the way. Check whether it is safe to climb Moel Siabod before you set off in the dark.
See which peaks have the best chance today on the UK cloud inversion forecast, or for the Snowdonia / Eryri mountains area.
Questions about Moel Siabod
Will there be a cloud inversion on Moel Siabod tomorrow?
Peakify gives Moel Siabod a 0% chance of a cloud inversion on Monday 19 October (Low), best around 04:00.
When is the best time to see a cloud inversion on Moel Siabod?
At or just before sunrise after a calm, clear night, most often in autumn and winter when nights are long and high pressure settles in. Fog in the valleys usually burns off as the sun warms them, so be on the top early.
How is the Moel Siabod cloud inversion chance worked out?
Each morning is scored at its best pre-dawn hour from wind at summit height, stability aloft, moisture in the valley, dry air above the summit, a clear night, pressure, overnight cooling and whether the top of the fog stays below the 872 m summit.