How to read this
Every weather app tells you it will be eight degrees and windy on the summit. None of them tell you the thing you actually want to know, which is whether you will be able to see anything when you get there.
The one idea
Cloud has a bottom. On a typical Lakeland day it might sit at 700 metres: below that line you can see for miles, above it you are inside a grey nothing with fifteen metres of visibility. That line is called the cloud base, and where your summit sits relative to it decides whether the day was worth the drive.
This site works out the cloud base for every Munro and every Wainwright, twice a day, and tells you which side of it each summit falls on. That is the whole product. Everything else on the page is in service of that one number.
Why other forecasts get summits wrong
Weather models describe the ground as a smooth surface. Even a good one, at two kilometres between grid points, cannot see a mountain properly: it shaves about sixty metres off Ben Nevis and far more off anything sharp. So when an app shows you conditions for a summit, it is usually describing a hill several hundred metres shorter than the one you are standing on, which is why it reads too warm, too calm and too clear.
This site ignores the model’s own ground level. It takes the profile of the atmosphere above each hill, level by level, and reads off the conditions at the summit’s real surveyed height. Same data, honest altitude.
Why it is a chance and not a promise
Cloud is rarely all or nothing. When cover is broken, a summit sits in and out of it for an afternoon, and the honest answer is a chance rather than a verdict. So the number is the odds of a view during walking hours, averaged across them rather than taken from the single luckiest one.
Where the forecast is genuinely borderline the site says so instead of picking a side. Could go either way is a real answer, and it is the one every other weather app refuses to give.
Why the hills are grouped by height
On a poor day the small fells are clear and the big ones are not, so ranking every hill together would simply list the shortest first. Grouping by height band tells you the thing that actually matters: the tops are in cloud, but there is still a walk worth doing lower down.
This forecast has not yet been checked against what actually happened on the hill. The method is sound, but until it has been scored against real observations over a season, treat it as an interesting second opinion and nothing more. It is not a mountain safety service, and it will never replace MWIS or, in winter, SAIS.
What the words mean
- Cloud base
- The height of the bottom of the cloud, above sea level. Compare it against a summit height to know which side of it you will be.
- Cloud top
- The height of the top of the same layer. If a summit is above it, you are looking down on cloud rather than standing in it.
- Inversion
- Cold air trapped in the valleys with warmer air above, so cloud fills the glens and the tops stand clear. Rare, and worth setting an alarm for.
- Freezing level
- The height at which the air reaches zero. Above it, rain becomes snow and wet rock becomes ice.
- Chance of a view
- The odds the summit is out of cloud, averaged across the hours people are actually up there rather than the single luckiest hour of the day.
- Sun azimuth
- The compass bearing the sun rises and sets on. For photographers, knowing where the light comes from matters as much as when.