Crib y Ddysgl cloud inversion forecast

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 Crib y Ddysgl is 19% (Low), confidence high. Summit air no warmer than the valley: no inversion in the model; no inverted layer in the profile near the summit.

Cloud inversion forecast for Crib y Ddysgl, next 7 days

Ratings: Excellent 70% or more, Good 50%, Marginal 30%, Low under 30%.

What is driving this morning's chance

• Warm air at summit height: Summit air 3.5°C colder than the valley (holding it back)
• Wind at summit height: 14 mph (neither helps nor hinders)
• Inverted layer: No inverted layer (-0.8°C) below 1465m (holding it back)
• Summit above the cloud: Cloud 0% at summit height (helps)
• Dry air at summit height: Humidity 61% at summit height (helps)
• Valley moisture: Dew point spread 3.0°C, low cloud 0% (holding it back)
• Clear night: Mid/high cloud 0% overnight (helps)
• Calm valley: 4 mph in the valley (helps)
• High pressure: 1037 hPa (sea level) (helps)
• Overnight cooling: Valley 1.0°C colder by dawn (holding it back)
• Fog top vs summit: Fog top ~105m, summit 1065m (helps)
• Night length: 13.5 hours of darkness (helps)

How the Crib y Ddysgl 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 (1,065 m) with the lowest ground within 6 km (55 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 Crib y Ddysgl the summit stands about 1,010 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 Crib y Ddysgl

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Play the 3D model peakify.ai

18:00 · Evening

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.

EveningNightDawnMorning

What if…

Reset

Wind on the tops

4 mph

Cloud overnight

0%

Moisture in the valley air

95%

A sea of cloud is likely: calm, clear and damp.

Read it step by step

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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 Crib y Ddysgl

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 Crib y Ddysgl 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 Crib y Ddysgl

Will there be a cloud inversion on Crib y Ddysgl tomorrow?

Peakify gives Crib y Ddysgl a 0% chance of a cloud inversion on Monday 19 October (Low), best around 05:00.

When is the best time to see a cloud inversion on Crib y Ddysgl?

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 Crib y Ddysgl 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 1,065 m summit.