ParaglidingWX — Thermal Map

Thermal Map

Thermal Map uses the same map time slider as Wind Map. Each coloured cell is the mean parcel buoyancy (°C) averaged from ground level through the lowest 200 m AGL at that grid point—the same parcel physics as the windgram Parcel buoyancy tab, collapsed to one number per map cell. It is a coarse planning hint, not a substitute for reading the full profile at a site.

Models and data

The overlay follows the toolbar Model dropdown (not the windgram model). It is available on HRDPS ~1 km (West), HRDPS 2.5 km, and HRRR ~3 km (CONUS) when those models use the hosted zarr map grids (see Weather models). Open-Meteo-only models do not drive the Thermal Map.

For those zarr models, buoyancy is usually fetched precomputed from /api/…-thermal-buoyancy-grid (server-side parcel integration with model orography / DEM). The same formula is also implemented in the browser for legacy paths that still pull raw isobaric fields and sample Terrarium DEM per cell.

Parcel calculation

At each map cell and forecast hour, a surface air parcel is lifted from terrain elevation using:

• 2 m temperature and relative humidity from the model for the surface parcel starting point.
• Isobaric temperature and RH on the model’s pressure levels for the environmental profile.
• LCL (lifting condensation level) from surface T and dew point; below the LCL the parcel cools at the dry adiabatic lapse rate (9.8 °C/km); above the LCL it cools at the saturated adiabatic lapse rate (moist-adiabatic, humidity-dependent).
• Entrainment — a small fractional mixing term (2×10⁻⁴ m⁻¹, ~20% per km) erodes buoyancy as the parcel rises, same as the windgram Parcel buoyancy tab.
• Virtual temperature — buoyancy at each height is T_{virtual,parcel} − T_{virtual,environment} in °C (moisture included).

The map value is the arithmetic mean of that buoyancy profile from terrain MSL to terrain + 200 m MSL, sampled at evenly spaced heights (endpoints included). Terrain comes from Mapzen Terrarium DEM tiles on client paths, or model orography on the server grid.

Colour scale

Colours match the windgram Parcel buoyancy tab (warm = still rising, cool = capped or stable):

• > +4 °C — very strong
• +2 to +4 °C — strong
• +0.5 to +2 °C — good
• 0 to +0.5 °C — marginal
• −1 to 0 °C — slightly stable
• −3 to −1 °C — stable
• < −3 °C — strong inversion

The raster is drawn with partial transparency so topography and the basemap remain visible underneath.

Limitations

• A single 200 m layer average hides inversion structure, cloud shading, valley breeze, and local terrain effects—use the windgram for the full vertical picture.
• Grid spacing follows the map model and zoom (coarser than 1 km in places); values are interpolated and lightly smoothed for display.
• Model thermals are not observations; compare with local knowledge and live conditions.

← Back to About