Weather and physical atmosphere

The singleton weather component describes atmospheric conditions independently of the environment component's Earth or authored sky. Clouds, precipitation and visibility are separate axes. Overcast need not rain; temperature never silently selects rain versus snow. There is no forecast service, browser geolocation request, or implicit host clock.

Add this entity to a molen/scene@3 manifest:

{
  "id": "conditions",
  "components": {
    "weather": {
      "atmosphere": {
        "temperatureK": 282.15,
        "pressurePa": 100200,
        "relativeHumidity": 0.95,
        "windVelocity": [7, 0, 3],
        "referenceAltitude": 0,
        "gasConstant": 287.05
      },
      "clouds": { "coverage": 0.98, "density": 0.95, "baseAltitude": 1600, "thickness": 1200, "scale": 16000 },
      "precipitation": { "kind": "rain", "intensity": 0.7 },
      "visibility": 4500,
      "seed": 1
    }
  }
}

One weather entity is supported: the first entity carrying the component wins. Changes, component removal and keyframe replacement propagate through the snapshot/delta path. Weather participates in state hashes, checkpoints and replays.

Units and profiles

WeatherData, weatherSchema, resolveWeather, WeatherProfile and weatherProfile are exported by @bendyline/molen-schema. The client re-exports the types, resolver and presets. weatherProfile('sunny' | 'partly-cloudy' | 'overcast' | 'rain' | 'snow' | 'fog') returns fresh editable data. resolveWeather(data) supplies defaults and copies nested values. Validate external data with weatherSchema.parse before using it in typed code.

Parameter	Meaning
atmosphere.temperatureK	Kelvin at the reference altitude; default 288.15 K (15 °C).
atmosphere.pressurePa	Absolute pressure at the reference altitude; default 101325 Pa. Zero allows vacuum.
atmosphere.relativeHumidity	0–1; stored for consumers, independent of precipitation.
atmosphere.windVelocity	World XYZ velocity in m/s; +X east and -Z north in Earth worlds. A velocity toward a direction, not a meteorological "from" bearing.
atmosphere.referenceAltitude	Absolute world Y where temperature/pressure apply, default 0 m.
atmosphere.gasConstant	Specific gas constant, default 287.05 J/(kg·K) for dry air; configurable for other worlds.
clouds.coverage, density	Independent 0–1 coverage and optical density controls.
clouds.baseAltitude, thickness, scale	Absolute base Y, layer separation, and horizontal pattern repeat size in meters.
precipitation.kind, intensity	Explicit none/rain/snow and 0–1 visual intensity; not calibrated mm/hour.
visibility	Approximate visual fade distance in meters, independent of clouds and precipitation.

Presets are starting points, not classifications that constrain these independent parameters.

Simulation and physics

Scripts read and update the ordinary component:

const current = molen.get('conditions', 'weather');
if (current) molen.patch('conditions', 'weather', {
  atmosphere: { ...current.atmosphere, temperatureK: 271.15 }
});

Typed systems use Weather, weatherOf(world) and sampleAtmosphere(data, altitude, gravity?) from @bendyline/molen-kernel. The sample contains temperatureK, pressurePa, densityKgM3, relativeHumidity and windVelocity. It uses deterministic math, ideal-gas density pressure / (gasConstant * temperature) and an isothermal hydrostatic pressure profile. Gravity defaults to 9.80665 m/s². Humidity is informational: there is no moist-air correction, temperature lapse rate or fluid simulation.

installAircraft reads the world's weather each physics tick. Wind changes airspeed, and temperature/pressure change aerodynamic density. Explicit AircraftEnvironment.weather overrides the component; explicit wind overrides its wind. stepAircraft accepts the same optional weather value. Without weather, aircraft retain their existing altitude-density model exactly. Ground friction and character temperature exposure are not implicitly changed.

Rendering and clocks

The normal client mirrors weather automatically. Standalone hosts can call:

import { weatherProfile } from '@bendyline/molen-client';
viewer.renderer.setWeather(weatherProfile('partly-cloudy'));
viewer.renderer.setWeatherTimeOverride(20); // absolute preview seconds
viewer.renderer.setWeatherTimeOverride(undefined); // resume simulation time
viewer.renderer.setWeather(undefined); // remove weather effects

Clouds use two wind-advected procedural layers at absolute world altitudes. They occlude celestial bodies, tint through dusk/night, and reduce direct lighting. Rain and snow share a bounded camera-local particle batch with wind drift. Seed/time sampling is deterministic; floating-origin changes preserve world alignment. No simulation random numbers are consumed.

Standard materials work on WebGL and WebGPU, with up to two cloud draws and one precipitation draw. Textures generate locally when needed; sunny startup generates no cloud noise and allocates no particle batch. Animation follows simulation time independently of a sky-time preview override, so pauses and captures remain repeatable.

Weather visibility temporarily limits scene fog; a nearer authored terrain fog limit and its clear foreground are preserved when weather visibility reaches beyond that limit. Removing weather restores the authored fog and lighting. Clouds are layered surfaces, not ray-marched 3D volumes: flying through a layer does not simulate condensation. Precipitation is a local visual field, including above cloud layers, without roof/terrain collision, splash, accumulation or wet-material response.

World explorer

The Weather dropdown includes Sunny / clear, Partly cloudy, Overcast, Rain, Snow and Fog. Weather details mixes cloud coverage/density/altitude, precipitation, intensity, visibility, temperature, pressure and wind, marking the profile Custom. Temperature and pressure controls refer to sea level, displayed in °C and hPa and stored in kelvin and pascals. The same component is installed in the explorer's vehicle/aircraft simulation.

Use ?weather=rain or another preset id. Custom edits are session-local. ?freeze=1 freezes weather motion for comparisons. The day slider changes lighting without moving the camera or seeking precipitation animation. The legacy ?sky=daylight path stays weather-free unless a weather profile is explicitly requested.