How the score works
Nobody publishes measured rock friction, so this score can't be checked against ground truth. It encodes what Bleau regulars already know and runs it every hour for the whole forest. Every constant behind it is published at /api/v1/meta/modelso you can argue with it instead of taking it on faith.
1One score, for the forest
The first question is whether to make the trip at all, from an hour away in Paris or six from Belgium. So the headline is one number for Fontainebleau as a whole: roughly what a well-picked spot will feel like, because you choose where to go once you're there. It isn't the average, which would undersell days you can plan around, and it isn't the single best block either, which would let one freak reading speak for the whole forest.
2Five areas, one score
Under the headline sit the five places people actually name: Fontainebleau, Bois le Roi, Milly, Nemours and Buthiers. Each is scored twice, once for its open blocks and once for its wooded ground. Rain is the reason for the split. A shower can soak Milly and miss Nemours, while temperature, humidity and wind barely change across the forest. Which boulder to walk to is your call once you arrive, so the app never names one.
3Rock temperature, not air temperature
Sun isn't scored on its own. It works through the temperature of the rock face, and that drives everything else. A south face in low winter sun can run 8 °C above the air while a north face sits at air temperature, and sandstone lags both by a few hours. Cold and sunny scores well, warm and sunny scores badly, and both come out of the same equation.
4Two layers of moisture
One wetness number can't describe the state that breaks holds: a surface dried by the wind over a core that's still soaked. The rock looks climbable and it isn't. So the model tracks a fast surface layer, which decides how greasy the holds feel, and a slow core, which decides whether it's safe to climb at all. Drying slows down as the rock dries, which is why a real soaking takes a day or more to clear.
5Friction
Four things decide whether skin sticks: how cold the rock is, how far it sits above the dew point, how thirsty the air is, and how much wind reaches it. Colder is always better for friction, right down past freezing, which is why people save their hardest projects for sub-zero days. What a freezing day costs you is comfort, and that's scored separately. Heat is the exception. Above about 26 °C it drags the whole friction figure down, because a scorching day can be bright, dry and breezy and your skin still sweats. Wind matters more than most people think. It strips the humid layer off the holds, which is why a humid, windy day can climb better than a still, dry one.
6Comfort, kept separate
At −6 °C the rock is superb and your fingers are useless. Comfort gets its own score with a light weight, so brutal days lose a few points without pretending the friction is bad.
7Wet rock is a gate, not a penalty
Wet Bleau sandstone breaks, and broken holds don't grow back. So wetness isn't one factor among many. Past a threshold the ground is marked unsafe and gets no score at all. Perfect temperature and a perfect breeze don't outvote it.
8Sessions, not instants
Hourly scores are grouped into sessions that are good in their own right and close to the best the day offers, then trimmed to daylight. Daylight comes from the real sunrise and sunset: a December day ends at 16:56 and a June one at 21:54, and that difference is most of the question. That's why you get 08:00–10:30 instead of one number for the whole day.
9The day, in a sentence
The verdict reads the whole day: when the good session starts, when it ends, and what ends it, whether that's rain, heat or the dark. Its quality word comes from the hours worth turning up for, so a good morning followed by rain is still a good morning. A single brilliant hour in a mediocre day is called a window, not a good day. When the wooded half of the forest is clearly doing better than the open half, or the other way round, the verdict says so.
Every metric
- Score
- How good the rock should feel right now, 0–100. Friction does most of the work, comfort adjusts it, and surface wetness scales the whole thing down.
- Rating
- The same score in half-star steps, for a quicker read.
- Friction
- How well skin should stick. Driven by rock temperature, how far the rock sits above the dew point, how thirsty the air is, and wind.
- Comfort
- How pleasant it is to be out there. Kept separate from friction so a −6 °C day doesn't score as perfect: the rock is superb, your fingers are not.
- Dryness
- How dry the rock surface is. It scales the whole score rather than adding to it: dry rock is what lets the other factors count.
- Rock temp(°C)
- Estimated temperature of the rock face, not the air. Sun on the face warms it well above the air, a clear night cools it below, and sandstone lags both by a few hours. Colder grips better, with no lower limit: 10–20 °C is good, 0–10 °C is excellent in dry air, and below freezing is send temps. Above 30 °C nothing saves it.
- Dew point(°C)
- The temperature at which moisture starts condensing out of the air.
- Dew spread(°C)
- Rock temperature minus dew point. A better humidity signal than relative humidity. Above 5 °C the holds feel dry; at zero or below the rock is condensing and nothing rescues it.
- Air dryness(kPa)
- Vapour pressure deficit: how much more moisture the air could still absorb. High means the air is actively pulling water off the rock.
- Effective wind(km/h)
- Wind actually reaching the rock, after tree cover and how exposed the spot is. Wind strips the humid layer off the holds, which is why a humid, windy day can climb better than a still, dry one.
- Surface wetness
- How wet the outer skin of the rock is, 0 dry to 1 saturated. Responds within hours and decides how greasy the holds feel.
- Core wetness
- How wet the rock is inside, 0 dry to 1 saturated. Responds over days. This is the safety number: damp sandstone breaks even when the surface feels dry.
- Confidence
- How much to trust this forecast. Falls with distance into the future and when the weather models disagree with each other.
- Faces
- The direction the main boulders face. Decides how much sun the rock gets, and so how fast it dries and how warm it runs.
- Tree cover
- 0 is open plateau, 1 is dense forest. Trees keep off some rain but also block wind and sun, so wooded ground holds moisture far longer.
- Wind exposure
- 0 is a sheltered hollow, 1 is an exposed ridge. Decides how much wind reaches the rock.
- Seepage
- 0 drains straight away, 1 stays damp for days. Poorly drained ground keeps moisture in the rock long after the rain stops.
A note on wet rock
Bleau sandstone is fragile when it's damp. Climbing on it breaks holds for good, and the outer skin can feel dry while the rock underneath is still soaked. When the app says somewhere is too wet, it isn't grading your session. It's about not wrecking the problem for everyone who comes after you.