Why usable lift can sit above the boundary layer
The solid usable-lift line on this project’s charts often rides above the dashed boundary-layer line. If both lines meant “thermal top,” one of them would have to be wrong. Neither is — they answer different questions, and the trail to why runs through Dr Jack’s own caveats, a 1980 aircraft campaign over the ocean, and a coefficient choice made in canadarasp’s source.
“Hcrit” names a family of methods, not one number
Dr. John “Dr Jack” Glendening’s BLIPMAP documentation defines Hcrit as the height where an average dry updraft falls below 225 ft/min — about 1.14 m/s — and presents it as a practical maximum thermalling height over flat ground. He also flags the limits himself: the assumptions lack quantitative validation, and boundary-layer top may work better over complex terrain.
canadarasp inherits the name and the idea of a sink threshold, but its meteogram code asks a different question: how high might the strongest core still beat a 1 m/s sink rate? That is the number this project ports. Same name, different quantity — “Hcrit” names a family of methods, not a portable value that every forecast system computes identically.
How much the strongest-core choice matters
Lenschow and Stephens fitted a normalized mean-updraft shape from aircraft observations. canadarasp applies its strongest-core variant before finding the sink-threshold crossing. The forecast derivations define the current formula, constants, fallback, and cloud cap. See for yourself below: one atmosphere, three sink thresholds — the 1.0 m/s panel reproduces the published series, and the other two show how much the top owes to that single parameter.
One atmosphere, three sink thresholds
Changing the sink threshold moves the strongest-core crossing without changing the parcel boundary layer or cloud base.
Three views of the same convective cycle at sink thresholds 0.7, 1.0, and 1.3 metres per second. Only the usable-lift series changes.
Two crossings, two kinds of physics
The boundary-layer line is a thermodynamic crossing: lift a dry surface parcel and find where it is no
longer warmer than the model environment. The usable-lift line is a kinematic crossing: evaluate a
modelled core until its vertical speed falls to the sink threshold. A strong core carries momentum
into the entrainment zone and the canadarasp curve is evaluated beyond z/D = 1, so its threshold can
land above the parcel-derived boundary-layer top. Smoothing cloud base and usable-lift top separately
can also change the hour-to-hour gap.
So the crossing is not a contradiction — but the solid line is not “more correct” either. The 4.0 coefficient is a pragmatic extrapolation from an average profile. The underlying 1980 observations came from aircraft legs over the ocean during AMTEX, not from paragliders centring thermals in a mountain valley. The code is precise; the physical transfer is an assumption.
What remains uncalibrated
Neither coefficient establishes a calibrated flight ceiling for mountain soaring. The archive can be paired with IGC tracks to measure bias by site, hour, wind regime, and pilot population; that study has not been done. The chart therefore names the ported quantity usable-lift top, not predicted maximum altitude. Reading a Meteogram keeps the boundary-layer and usable-lift lines as different questions.
Primary trail: Dr Jack’s BLIPMAP parameter documentation, Lenschow & Stephens (1980), DOI 10.1007/BF00122351, and the canadarasp implementation.