The mountain the model sees

Every profile document carries two elevations for the same place: site.altitudeM, the surveyed launch, and site.modelElevationM, the elevation of the model’s own terrain at that grid point. At the launches in this project’s catalogue the two routinely disagree by tens to hundreds of metres — and that disagreement is not an error to fix but a fact to read. This article is about reading it: where the gap comes from, and what the static site context document adds about the real mountain — the relief the launch sits in and the ground its thermals feed on.

Why the model’s mountain is smoother than yours

A weather model does not know your launch. It knows one terrain height per grid cell, and a 2.5–10 km cell averages the ridge into the valleys beside it, so a launch on a summit almost always sits above the model’s terrain. The effect is resolution, not model quality — the same smoothing appears in elevation datasets themselves: the 90 m Copernicus DEM already reads ridge-top launches lower than its 30 m sibling, and a weather grid a hundred times coarser smooths far harder. The measured sizes of those effects are in the source facts of the reference page.

How well is the real mountain known? Within metres. For each catalogued launch, three independent answers — the surveyed elevation, a bare-earth lidar or national DTM, and the 30 m Copernicus surface model — land on a scale where a few metres are visible:

Generated from site-context.json and sites.json. produced using Copernicus WorldDEM-30 © DLR e.V. 2010-2014 and © Airbus Defence and Space GmbH 2014-2018 provided under COPERNICUS by the European Union and ESA; all rights reserved. Contains information licensed under the Open Government Licence – British Columbia.. Contains information licensed under the Open Government Licence – Canada..

surveyed launch bare-earth DTM GLO-30 surface (DSM)model terrain (site.modelElevationM): live per profile, off these scales

Elevations are read from site-context.json and sites.json on every build. The weather model's own terrain is deliberately absent: it is live data — read site.modelElevationM from any profile and place it against this ladder yourself.Units elevation m MSL · deltas vs the surveyed launch

The one number deliberately missing from that ladder is the model’s: site.modelElevationM is live data that changes with the model you pick, so it stays in the profiles. Open any current profile document and put its site.modelElevationM against the ladder — that distance is the smoothing, per model, at your launch.

What the gap means on a windgram

A windgram draws both worlds on one altitude axis, and the gap sits between them:

The launch marker is the surveyed launch — site.altitudeM, the ladder’s diamond.
“Surface” is the model’s ground — surface temperature, heat fluxes, w*, surface wind, and gusts are all referenced to the model’s smoothed terrain at site.modelElevationM, which can be hundreds of metres below the marker. The pipeline also uses model elevation, not launch elevation, for pressure-level filtering and the model-column derivations (why).

So “usable lift reaches the launch marker” is a statement that crosses the gap: the model’s thermal column, rooted at model ground, must grow tall enough to pass a launch the model cannot see. Reading a windgram builds the full checklist on that footing.

Relief percentile: trigger and topology

Knowing the launch’s elevation is not the same as knowing what it sits on. The context document’s relief discs rank the launch against every piece of terrain within 1, 3, and 10 km — and the percentiles, read together, are a compact topology:

Generated from site-context.json. produced using Copernicus WorldDEM-30 © DLR e.V. 2010-2014 and © Airbus Defence and Space GmbH 2014-2018 provided under COPERNICUS by the European Union and ESA; all rights reserved.

terrain min–max within the disc launch elevation (GLO-30)pctl 100 = the launch is the disc's summit

Each bar is the min–max terrain elevation within that radius of the launch; the vertical line is the launch's GLO-30 elevation, and the percentile is its rank among the disc's terrain — 100 means the launch is the local summit.Units elevation m MSL · disc radii km · percentile rank

Two catalogued sites make the grammar clear:

Red Mtn (100 / 87 / 84) is a local summit inside bigger terrain: nothing within 1 km stands above the launch, while higher ground exists at 3 and 10 km. A summit launch is its own trigger — heated slopes fall away on all sides, thermals can arrive from any of them, and no single “the hill faces X” reading applies. Its aspectDeg says 61°, but on a summit that number is genuinely low-confidence — the reference caveats measure how ambiguous.
Flagpole (82 / 83 / 47) is a foothill in front of bigger terrain: it stands proud of its immediate surroundings, yet more than half the terrain within 10 km is higher. The launch works its own slope early, then the day belongs to the larger walls behind it.

The mid-slope sites read differently again: Dundee (80 / 79 / 57) and Erie (72 / 55 / 48) sit on the flanks of their terrain, where the downslope bearing is trustworthy — both face southwest (241° and 236°) — and the trigger is the heated slope below the launch rather than the launch itself.

Land cover: what the thermal catchment is made of

Thermals are made downstream of the ground: forest canopies hold heat back, clearcut, rock, and grass release it readily, and water kills it. The context document measures that character as class fractions of the 1 and 3 km discs around each launch:

Dundeelaunch pixel: grassland

1 km

97%

3 km

94.3%

Erielaunch pixel: grassland

1 km

89.4%10.6%

3 km

88.3%11.1%

Flagpolelaunch pixel: tree cover

1 km

99%

3 km

74.5%13.4%

Red Mtnlaunch pixel: built-up

1 km

87.5%12.2%

3 km

85.5%10%

Generated from site-context.json. © ESA WorldCover project 2021 / Contains modified Copernicus Sentinel data (2021) processed by ESA WorldCover consortium.

tree covergrasslandbuilt-upbare / sparsewatermoss / lichen

Fractions are the share of each disc under a class, from ESA WorldCover 10 m (2021). A class absent from a bar is genuinely zero there — the map is wall-to-wall. The launch-pixel class is one 10 m pixel and is fragile: read it beside the 1 km bar, never alone.Units fraction of disc area % · disc radii km

The catalogue’s clearest teaching case is Flagpole’s 3 km ring: 13% water — Kootenay Lake’s west arm sits next door. A large cold surface inside the thermal catchment is a lake-breeze engine and a thermal killer, and no wind-profile model at 2.5 km resolves what it does to a house thermal. The bars also show why the disc fractions outrank the single launch pixel: Red Mtn’s launch pixel reads built-up — resort infrastructure under the launch — while its 1 km disc is 88% tree cover. One 10 m pixel answers “what is under the ramp”; the fractions answer “what feeds the climb”.

Honest limits

The context is static geography from open data, each source with its licence and its measured error — the surface model includes canopy, the land-cover map is 76.7% accurate globally, vertical datums differ by decimetres, and summit aspects are ambiguous. All of it is stated, with dated verification stamps, in the site context reference, along with the document’s shape and the code to join it to a profile by slug. What the context can never do is change with the weather: it tells you what the mountain is, so the windgram can tell you what today’s atmosphere does with it.

The mountain the model sees

Why the model’s mountain is smoother than yours

Three answers within metres — and one far away

What the gap means on a windgram

Relief percentile: trigger and topology

Where the launch sits in its terrain

Land cover: what the thermal catchment is made of

What the thermal catchment is made of

Honest limits