Volumetric fog

#The dials

weather.set({ fog: 0.4 });                    // amount
weather.set({ fog: 0.62, fogHeight: 0.3 });   // low ground fog, like foggyDawn
weather.set({ fogLight: 0.5 });               // dimmer in-scattering

Each frame FogSystem turns these (plus rain and snow) into physical values:

density = 0.0015 + fog² × 0.11 + rain × 0.01 + snow × 0.012     (1/m, at the fog base)
height  = (5 + fog × 12 + snow × 8) × fogHeight                   (m, exponential falloff)

weather.fog.visibility reports 3 / density in metres, the distance at which 95 % of the light is gone at the fog base. The demo HUD shows it as VIS.

#With weather.render()

The pipeline raymarches the fog at reduced resolution. Steps and resolution come from the quality tier: 20 steps at 0.35× on low, up to 56 steps at 0.75× on ultra. Along the way it adds:

• Light shafts. With a shadow-casting shadowLight (see below), the march samples its shadow map, so sunlight and moonlight break into shafts around your geometry.
• Lamp halos. The localLights glow in the fog.
• Lightning. The flash lights the fog volume around the bolt.
• Drift. The fog noise drifts with the wind.

weather.setLighting({ shadowLight: sun });   // a DirectionalLight with castShadow = true
weather.fog.rays = false;                    // shafts off (the demo's "Fog rays" toggle)
weather.fog.maxDistance = 120;               // march distance, default 170 m

rays = falserays = true

#With renderer.render()

When you don't call weather.render(), the fog is written to scene.fog as a THREE.FogExp2. Its density is matched to the same 95 % visibility, and its colour is the fog ambient × fogLight. Every built-in material fogs with it.

weather.fog.sceneFog = 'auto';   // default: FogExp2 only while render() isn't used
weather.fog.sceneFog = true;     // always drive scene.fog
weather.fog.sceneFog = false;    // never touch scene.fog (e.g. next to NatureGL Water)

Once you call weather.render() for the first time, 'auto' removes the FogExp2 it created, and the volumetric fog takes over.

#Fog colour

The fog is lit by your light, not given a colour. setLighting feeds it:

• skyHorizon and skyZenith set the fog ambient (horizon × 0.4 + zenith × 0.15, plus a little of the key light). They only take effect as a pair: one without the other is ignored.
• Otherwise ambient is used (ambient + a little of the key light).
• The key light (sunDirection, sunColor, sunIntensity) makes it glow toward the sun.

Next to NatureGL Sky, the ambient you pass from the sky drives the fog colour. See With other packs.