The sounding

The sounding is the package’s second chart family. It draws one forecast hour of a validated profile document as a vertical profile. The Meteogram connects a day of columns through time, and the sounding opens up a single column. It shows temperature and dew point against height, a lifted parcel beside them, and a wind-barb ladder in the right margin. The derived heights (boundary layer top, cloud base, usable lift top) appear as horizontal marks. The sounding has its own subpath and works in the same two steps as the Meteogram: a renderer-independent scene graph, then deterministic SVG. This page is the reference for that subpath. If you have not rendered a profile before, start at Render a first Meteogram.

What this chart is

This is a profile of the flyable band, and it stops where the published column stops. Both axes are linear: height in metres MSL, with the floor at the model elevation, and temperature. It is not a skew-T. It has no skewed temperature coordinate and no adiabat grid, because the input cannot fill one. A profile document carries only the levels its model publishes in the flyable band. The deterministic models in the catalogue publish nothing above 600 hPa, and ensemble models publish far fewer levels than that, so the upper half of a skew-T would be invented. The chart draws exactly the published column and prints where it ends. It draws and implies nothing above the top level.

The marks follow the same rule. A derived height that the document does not carry adds no mark, and an hour whose required medians are absent builds no scene at all.

Build the scene for one hour

buildSoundingScene(profile, options) selects the hour by instant, and options.validAt is the only selector. An instant the profile does not publish returns null rather than throwing. The scene echoes validAt back, so a Meteogram selection can drive a sounding next to it, and nothing public is keyed by hour index.

build-sounding.ts

import type { SiteForecast } from "@azohra/meteo.briefing/contract";
import { buildSoundingScene, renderSoundingSvg } from "@azohra/meteo.briefing/sounding";

export function soundingAt(profile: SiteForecast, validAt: string): string | null {
  const scene = buildSoundingScene(profile, {
    validAt,
    // Supply the launch elevation if you want the mark; the document
    // has none.
    launch: { elevationM: 1225.1 },
  });
  if (scene === null) return null; // the profile does not publish this instant
  return renderSoundingSvg(scene, { idPrefix: "club-sounding" });
}

To pair the sounding with a Meteogram, take the instant from the Meteogram’s own scene instead of formatting one yourself:

drive-from-meteogram.ts

import type { SiteForecast } from "@azohra/meteo.briefing/contract";
import { buildMeteogramScene } from "@azohra/meteo.briefing/meteogram";
import { buildSoundingScene } from "@azohra/meteo.briefing/sounding";

export function soundingForSelectedHour(profile: SiteForecast, timeZone: string) {
  const meteogram = buildMeteogramScene(profile, { timeZone });
  const validAt = meteogram.hourValidAts[meteogram.selectedHourIndex];
  return buildSoundingScene(profile, {
    validAt,
    // Pin the altitude axis to the Meteogram's own domain so the two
    // charts read against the same scale.
    floorM: meteogram.scales.floorM,
    topM: meteogram.scales.topM,
  });
}

By default the altitude domain follows the Meteogram’s rules over the whole profile plus the launch. The floor sits at the model elevation, and the top is padded above every level and every drawn derived height. The axis therefore stays still while a consumer scrubs through hours. floorM and topM override the domain, and widthPx and heightPx set the SVG size. overlays turns each drawn layer on or off: temperature, dewPoint, parcel, wind, boundaryLayerTop, cloudBase, usableLiftTop, and launch. All of them are on by default, and DEFAULT_SOUNDING_OVERLAYS exports the set.

Count the levels off the chart

The chart shows no more vertical resolution than the document carries.

• Every published level draws as a dot on the temperature and dew-point traces, with one more dot for the surface sample. A reader can count the model’s levels directly off the chart. The note under the plot states the count and where the column ends (5 published levels · top of column 2538 m).
• Segments between dots are straight, so the chart suggests no structure between levels that the model did not publish. The measured environment draws solid. Only the parcel trace is dashed, because it is the one derived trace rather than a published value.
• A 5-level ensemble column renders as five dots and four straight segments, with p25–p75 envelopes behind the traces and behind each ensemble-valued mark. The envelope is the members’ spread at the published levels, joined by the same straight segments as the median.
• The wind ladder is not thinned. It has one barb at the surface and one per published level, at that level’s drawn height. Feathers are 5, 10, and 50 km/h, as on the Meteogram.

The parcel trace and the LCL

The parcel trace lifts the hour’s surface parcel through the published levels. It rises dry-adiabatically to its lifting condensation level (LCL) and moist pseudo-adiabatically above it. The chart draws the parcel’s temperature next to the environment’s and marks the LCL on the trace when it falls inside the drawn band. Buoyancy at any height is the horizontal gap between the parcel trace and the temperature trace. readingAtAltitude reports it as a number, as a virtual-temperature difference so that moisture counts. For ensemble documents the parcel starts from the p50 member, and the parcel trace itself has no envelope.

Answer pointer positions

readingAtAltitude(scene, y) interpolates the column at a scene y. It returns temperature, dew point, dew-point depression, wind speed and direction, parcel temperature, and buoyancy. Each value is null above the published column or where the document carries no value. Interpolation is linear between published levels, which matches the straight segments on the chart, so a tooltip and the pixels agree. yForAltitude, altitudeForY, and xForTemperature expose the scales for consumer overlays.

Render SVG and the scene-derived key

renderSoundingSvg(scene, { idPrefix }) emits a self-contained SVG document with stable ordering and two-decimal geometry. Identical input produces identical bytes. Give each sounding on a page its own idPrefix.

The chart labels itself in place. Each trace prints its name in ink behind a short chip of line in the trace’s colour, at the surface end. Each altitude mark, and the LCL, prints its name and height next to its own line, on the half of the plot farther from the traces at that altitude. Both sets of labels are placed deterministically so they do not collide. Marks at the same height stack a minimum gap apart, and a label moved off its true height carries a leader tick back to it. Because of this, buildSoundingKeySpec(scene) keys only what the plot does not already label: the published-level dot, the ensemble envelope when one is drawn, and the calm circle when the wind ladder drew a calm level. That is three entries at most. Pass selfLabeled to add the self-labeling traces, marks, and LCL back into the key; the exported SOUNDING_SELF_LABELED is the complete set. renderSoundingKeySvg serializes the spec with the same stylesheet. Rebuild the key from the final scene after every overlay change.

Theming

The chart is styled by its own --meteo-sounding-* token family. SOUNDING_TOKEN_DEFAULTS holds the default values. SOUNDING_TRACE_TOKENS and SOUNDING_MARK_TOKENS map each drawn class to the token that colours it, for legends and focus styles. Quantities that appear on both charts keep the Meteogram’s colours (cloud base is the same colour on both), but the sounding reads only its own token family. Override tokens on an ancestor, as the SVG renderer page describes for the Meteogram, or pass stylesheet: null to supply all the class styling yourself. Colour is always paired with another encoding. The environment traces are solid and the derived parcel trace is dashed, every trace prints its name in ink next to a chip of line, marks differ by dash and printed label, and the dots and the note work with any palette.