Reading the wind stack: surface to 700 hPa
The API serves wind at the surface and at 925, 850, 800 and 700 hPa. Here is what each level tells a soaring pilot, and why the shear between them is its own field.
Most weather apps give you one wind: the surface wind, the one that decides whether a hill is on. Convek gives you that plus four levels above it - 925, 850, 800 and 700 hPa - and a shear field on top. This post is what each of those is for, because a stack of wind barbs is only useful if you know which one answers which question.
Start at the bottom. `surface_wind_speed_ms` and `surface_wind_dir_deg` are the WRF 10 m wind. This is the terrain-influenced wind: it has felt the hill, the valley, the coastline, so it is what decides launchability and ground drift. It is also the one that diverges most from the gradient, because near the surface friction and local terrain dominate.
Then the pressure levels, each at roughly a useful soaring altitude. `wind_925` is at about 750 m - the wind your low thermals drift in early in the day, and the level the convergence diagnostics are computed on. `wind_850` is about 1500 m, the heart of a typical UK thermal layer. `wind_800` is around 2000 m and `wind_700` around 3000 m - the top of a strong day, and the level where wave and the synoptic flow start to show through cleanly. Each comes as a `_speed_ms` and a `_dir_deg`.
The reason to look at more than one level is that the change with height is where the day's structure lives. Wind that veers (turns clockwise) with height usually means warm advection and often a workable day; backing wind can flag the opposite. A surface wind that disagrees sharply in direction with 925 hPa is a classic sea-breeze signature - the shallow marine flow underneath the gradient. And a thermal that drifts one way low down and another way higher up is harder to core and harder to leave, which you can read straight off the difference between 925 and 850.
Which is where the shear field comes in. `shear_surface_925_ms` is the magnitude of the wind shear between the surface and 925 hPa, served as its own field rather than something you have to derive. Shear matters for soaring because it tears thermals apart: a strong buoyancy signal under strong low-level shear gives you broken, scrappy, hard-to-use lift, while the same buoyancy under light shear gives you clean, organised cores. That is not just intuition - it is in the day rating, which uses a buoyancy-to-shear ratio (`wstar_ms` over the shear) as one of its scoring terms. A day can have a healthy `wstar_ms` and still rate lower because the shear is shredding it.
A practical pattern: a day with a light surface wind but a strong 850 hPa wind is the one to be wary of. It can look benign on the ground and on `wstar_ms`, then blow the tops of the thermals off as you climb into the faster air, and a downwind dash can leave you unable to get back. Cross-checking the surface against the levels above is how you catch that before you commit to it, not after.
All of these come back in every `/v1/site` and `/v1/forecast` response, and the surface and 925 layers are also available through the grid endpoint for map rendering. Full field list and units are on the API reference.