Cloud has a bottom.
Every mainstream forecast tells you whether it will rain. In the hills the question is a different one. On a typical day the cloud sits with its base at, say, 700 metres: below that line you can see for miles, and above it you are inside a grey nothing with fifteen metres of visibility. Which side of the line a summit falls on decides whether the day was worth the drive.
This site works out that line, the cloud base, twice a day for all 282 Munros and all 214 Wainwrights, and tells you which side of it each summit should be on. That is the whole product. Everything else on the page is in service of that one number.
The summit is under the base. You see the valley, the ridge, the next hill over. This is the view you drove for.
The summit is above the base. Fifteen metres of grey in every direction, and a compass bearing to find the cairn.
Cloud fills the valleys and the top stands clear above a white sea. Perhaps twenty mornings a year. Worth driving through the night for.
A chance, not a promise
The cloud base is estimated from atmospheric models, and it moves. A hill 20 m under the base is a coin toss; a hill 300 m under it is a fairly safe bet. That is why every hill gets a percentage rather than a verdict, and why the forecast says could go either way where it genuinely could.
Cloud is rarely all or nothing either. When cover is broken a summit sits in and out of it for an afternoon, so the number is the odds of a view during walking hours, averaged across them rather than taken from the single luckiest one.
Why other forecasts get summits wrong
A postcode forecast describes the valley floor, where the weather station is. The summit 900 m above it is usually in different air: colder, windier, and often inside cloud the valley never sees. Sunny intervals in Fort William can mean zero visibility on Ben Nevis, and on an inversion morning the reverse.
Weather models make it worse by describing 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. This site ignores the model’s own ground level, 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 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.
How much to trust it.
The forecast has now been scored against real observations: 16,745 summit-hours of it, checked against cloud base measured at airfields across Scotland, the Lake District and Wales, over sixty days. The results were not used to tune anything, which is the only way the numbers below mean anything.
It is not equally reliable in both directions, and saying so is more useful than a single confident claim.
When it says you will get a view
Right about nine times in ten. A high percentage is close to trustworthy. If the site says a summit will be clear, it usually is, and you can plan around it.
When it says the summit is in cloud
Right a bit more than half the time. A low percentage is barely better than a shrug. The forecast leans pessimistic: it would rather warn you off a good day than promise a view that is not there. Read a low number as poor odds, not a closed door.
Cloud base, in metres
Within 200 m about six times in ten when the cloud is genuinely low, which is when the question matters. It is worse in the band between 800 and 1400 m, where most Munro summits sit, and that is the current weak point.
What is still unknown
All of this was measured at airfields, because almost nobody measures cloud base on a summit. Air lifted over a mountain raises the cloud base, so a hill is a harder case than a runway. Every hour of it is also summer, and winter, when the inversions happen, is untested.
Glossary
- 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.
- Could go either way
- The summit is within the model’s own margin of error of the cloud top or base. The honest answer is that nobody knows, and the site says so rather than picking a side.
- Sun azimuth
- The compass bearing the sun rises and sets on. For photographers, knowing where the light comes from matters as much as when.
- Prominence, or drop
- How far you must descend from a summit before you can climb anything higher. It is what separates a hill in its own right from a bump on a ridge.