← Research

B/S does not transfer to the valley

The buoyancy/shear ratio is flatland doctrine with a long pedigree: divide the boundary layer’s convective velocity scale W* by the vector wind difference between the surface and the boundary-layer top, and low values warn that organized shear will tear thermals apart before a glider can work them. This project computes it exactly as its predecessors plot it, so pilots can compare numbers directly. This entry records the day the ratio’s hidden assumption surfaced at a mountain site, what the assumption is, and what the package now says about it.

The trace

A production consumer’s real reading — hrdps-continental at a 1,537 m Kootenays valley site, run 2026-08-09T00Z, for a classic light-wind summer Sunday (flyable nine to six, cloud base near 4,000 m, gusts under 22 km/h):

Every wind in that table is light. The 5 m/s of “shear” is almost entirely direction. By the flatland reading — where the strip’s 0–5 axis puts anything under 1 deep in the broken zone — this stellar day is condemned three times over, and so is essentially every good summer day at sites like it.

The assumption

The ratio’s shear term subtracts two wind vectors and calls the magnitude “shear”, which measures something dynamical only when both vectors sample the same flow. On flat ground they do: the 10 m wind is the bottom of the boundary layer’s own profile, and a large vector difference across the layer is a real gradient a thermal must climb through. In a mountain valley the model’s 10 m wind is something else — thermally driven circulation, anabatic flow that rotates with the sun and is decoupled from the flow at boundary-layer top. Subtracting a valley breeze from a ridge-top wind does not measure a gradient any parcel experiences; it measures the angle between two different wind systems, and on the best days that angle is large by construction, because the valley wind follows the heating while the flow aloft follows the synoptics. The ratio is not noisy at such sites. It is structurally pinned to 0.3–0.7 precisely when conditions are best — a doctrine answering the wrong question with full confidence.

What the package now says

Three dispositions, in the order the evidence supports them:

1. The assumption is documented at the quantity’s home. surfaceToBoundaryLayerShearMs and buoyancyShearRatio in the derivation API now state the same-air-mass assumption, carry this trace, and point terrain-driven consumers at the height-resolved windShear field — which measures shear layer by layer within the column and never straddles the decoupling, so it survives terrain the way launch-relative heights survive elevation bias. No classed-cell treatment (“good/broken” doctrine) ships on B/S.
2. Unbounded is not missing. Zero shear makes the ratio infinite — buoyancy fully unopposed, the best possible reading — and the strip used to render it as the same gap as “no ratio computable”, which read as missing data. An unopposed hour now draws its own classed cell (wg-bs-unopposed); the gap again means only what a gap should.
3. The tempting repair stays unproven. Re-anchoring the shear vector’s lower end above the valley circulation (the first published level, or launch height) might restore the ratio’s meaning at mountain sites — and might not, and would quietly make the number model-dependent, since models publish different lowest levels. That variant ships nothing until it is prototyped against days pilots actually rated, the same bar every threshold in analyze had to clear.

The general lesson is the same one the stability ramp taught from a different direction: a quantity inherited from a flatland lineage carries its terrain assumptions silently, and the fix starts with saying the assumption out loud at the quantity’s one home.