Imported from HammerOfSteel/tomes_towers_and_transmutation (.agents/skills/procedural-weather-threejs/SKILL.md). Install upstream with npx skills add HammerOfSteel/tomes_towers_and_transmutation --skill procedural-weather-threejs. Copyright stays with the author.

Procedural Weather

Dynamic, layered weather effects in Three.js — GPU particle precipitation, volumetric fog, lightning, and smooth state transitions.

Architecture Overview

┌──────────────────────────────────────────────────────┐
│                Weather System                         │
│                                                      │
│  WeatherController (state machine)                   │
│    ├── current state + target state                  │
│    ├── transition progress (0→1)                     │
│    └── drives all subsystems:                        │
│                                                      │
│  ┌─ Precipitation ──────────────────────────────┐    │
│  │  GPU particles: rain, snow, hail             │    │
│  │  Ground splashes, accumulation               │    │
│  └──────────────────────────────────────────────┘    │
│  ┌─ Atmosphere ─────────────────────────────────┐    │
│  │  Fog, mist, dust, haze                       │    │
│  │  Volumetric (WebGPU) or exponential (WebGL)  │    │
│  └──────────────────────────────────────────────┘    │
│  ┌─ Electrical ─────────────────────────────────┐    │
│  │  Lightning bolts, sheet flashes              │    │
│  │  Cloud-internal illumination                 │    │
│  └──────────────────────────────────────────────┘    │
│  ┌─ Optical ────────────────────────────────────┐    │
│  │  Rainbows, aurora, god rays                  │    │
│  │  Wet lens, frost overlay                     │    │
│  └──────────────────────────────────────────────┘    │
│  ┌─ Environment ────────────────────────────────┐    │
│  │  Sky darkening, light color shift            │    │
│  │  Wind direction (shared across systems)      │    │
│  └──────────────────────────────────────────────┘    │
└──────────────────────────────────────────────────────┘

Renderer Setup

import * as THREE from 'three';

async function createRenderer(canvas) {
  let renderer, gpuAvailable = false;
  try {
    const WebGPU = (await import('three/addons/capabilities/WebGPU.js')).default;
    if (WebGPU.isAvailable()) {
      const { default: WebGPURenderer } = await import(
        'three/addons/renderers/webgpu/WebGPURenderer.js'
      );
      renderer = new WebGPURenderer({ canvas, antialias: true });
      await renderer.init();
      gpuAvailable = true;
    }
  } catch (e) {}
  if (!renderer) {
    renderer = new THREE.WebGLRenderer({ canvas, antialias: true });
    renderer.toneMapping = THREE.ACESFilmicToneMapping;
  }
  renderer.setSize(innerWidth, innerHeight);
  renderer.setPixelRatio(Math.min(devicePixelRatio, 2));
  return { renderer, gpuAvailable };
}

Shared Wind System

Wind drives all weather subsystems — precipitation angle, fog drift, debris direction.

class WindSystem {
  constructor() {
    this.direction = new THREE.Vector3(1, 0, 0.3).normalize();
    this.baseSpeed = 5;       // m/s
    this.gustSpeed = 0;
    this.gustFrequency = 0.5;
    this.turbulence = 0.1;
    this._time = 0;
  }

  update(dt) {
    this._time += dt;
    // Gusts: low-frequency intensity variation
    const gustEnvelope = Math.sin(this._time * this.gustFrequency) * 0.5 + 0.5;
    this.gustSpeed = gustEnvelope * this.baseSpeed * 0.6;
  }

  get speed() { return this.baseSpeed + this.gustSpeed; }

  // Wind force vector for particle displacement
  get force() {
    const s = this.speed;
    const turb = new THREE.Vector3(
      Math.sin(this._time * 2.3) * this.turbulence,
      0,
      Math.cos(this._time * 1.7) * this.turbulence
    );
    return this.direction.clone().multiplyScalar(s).add(turb);
  }
}

Precipitation System

GPU particle system for rain, snow, and hail. Particles spawn in a box above the camera, fall with gravity + wind, and recycle when below ground.

Particle Buffer Layout

function createPrecipitationBuffers(maxParticles) {
  // Each particle: vec4(x, y, z, life) + vec4(vx, vy, vz, size)
  const positions = new Float32Array(maxParticles * 4);
  const velocities = new Float32Array(maxParticles * 4);
  return { positions, velocities, count: maxParticles };
}

Rain System

class RainSystem {
  constructor(scene, options = {}) {
    this.scene = scene;
    this.count = options.count ?? 50000;
    this.spawnRadius = options.spawnRadius ?? 40;
    this.spawnHeight = options.spawnHeight ?? 30;
    this.dropLength = options.dropLength ?? 0.3;
    this.intensity = options.intensity ?? 1.0; // 0-1

    this._buildGeometry();
  }

  _buildGeometry() {
    // Each raindrop: a short line segment (2 vertices)
    const positions = new Float32Array(this.count * 6); // 2 verts × xyz
    const randoms = new Float32Array(this.count * 2);   // per-drop seed + phase

    for (let i = 0; i < this.count; i++) {
      const x = (Math.random() - 0.5) * this.spawnRadius * 2;
      const y = Math.random() * this.spawnHeight;
      const z = (Math.random() - 0.5) * this.spawnRadius * 2;

      // Top vertex
      positions[i * 6]     = x;
      positions[i * 6 + 1] = y;
      positions[i * 6 + 2] = z;
      // Bottom vertex (offset by drop length)
      positions[i * 6 + 3] = x;
      positions[i * 6 + 4] = y - this.dropLength;
      positions[i * 6 + 5] = z;

      randoms[i * 2]     = Math.random();     // seed
      randoms[i * 2 + 1] = Math.random();     // phase offset
    }

    const geometry = new THREE.BufferGeometry();
    geometry.setAttribute('position', new THREE.BufferAttribute(positions, 3));
    geometry.setAttribute('aRandom', new THREE.BufferAttribute(randoms, 2, false, this.count));

    this.material = new THREE.ShaderMaterial({
      uniforms: {
        time:       { value: 0 },
        intensity:  { value: this.intensity },
        windForce:  { value: new THREE.Vector3() },
        gravity:    { value: -15.0 },
        spawnHeight: { value: this.spawnHeight },
        spawnRadius: { value: this.spawnRadius },
        dropLength: { value: this.dropLength },
        opacity:    { value: 0.35 },
        cameraPos:  { value: new THREE.Vector3() },
      },
      vertexShader: RAIN_VERT,     // See references/weather-shaders.md
      fragmentShader: RAIN_FRAG,   // See references/weather-shaders.md
      transparent: true,
      depthWrite: false,
      blending: THREE.AdditiveBlending,
    });

    this.mesh = new THREE.LineSegments(geometry, this.material);
    this.mesh.frustumCulled = false;
    this.scene.add(this.mesh);
  }

  update(time, wind, cameraPos) {
    this.material.uniforms.time.value = time;
    this.material.uniforms.windForce.value.copy(wind.force);
    this.material.uniforms.cameraPos.value.copy(cameraPos);
    this.material.uniforms.intensity.value = this.intensity;
    // Recenter spawn box around camera
    this.mesh.position.x = cameraPos.x;
    this.mesh.position.z = cameraPos.z;
  }

  dispose() {
    this.scene.remove(this.mesh);
    this.mesh.geometry.dispose();
    this.material.dispose();
  }
}

Snow System

Snow uses Points with per-particle flutter and tumble. Larger particles, slower fall, more wind influence.

class SnowSystem {
  constructor(scene, options = {}) {
    this.scene = scene;
    this.count = options.count ?? 20000;
    this.spawnRadius = options.spawnRadius ?? 50;
    this.spawnHeight = options.spawnHeight ?? 25;
    this.intensity = options.intensity ?? 1.0;

    this._build();
  }

  _build() {
    const positions = new Float32Array(this.count * 3);
    const seeds = new Float32Array(this.count * 3); // seed, phase, size

    for (let i = 0; i < this.count; i++) {
      positions[i * 3]     = (Math.random() - 0.5) * this.spawnRadius * 2;
      positions[i * 3 + 1] = Math.random() * this.spawnHeight;
      positions[i * 3 + 2] = (Math.random() - 0.5) * this.spawnRadius * 2;
      seeds[i * 3]     = Math.random();
      seeds[i * 3 + 1] = Math.random() * Math.PI * 2;
      seeds[i * 3 + 2] = 0.5 + Math.random() * 1.5; // size variation
    }

    const geometry = new THREE.BufferGeometry();
    geometry.setAttribute('position', new THREE.BufferAttribute(positions, 3));
    geometry.setAttribute('aSeed', new THREE.BufferAttribute(seeds, 3));

    this.material = new THREE.ShaderMaterial({
      uniforms: {
        time:        { value: 0 },
        intensity:   { value: this.intensity },
        windForce:   { value: new THREE.Vector3() },
        gravity:     { value: -1.5 },
        spawnHeight: { value: this.spawnHeight },
        spawnRadius: { value: this.spawnRadius },
        flutterAmp:  { value: 2.0 },
        opacity:     { value: 0.85 },
        pointSize:   { value: 3.0 },
        cameraPos:   { value: new THREE.Vector3() },
      },
      vertexShader: SNOW_VERT,     // See references/weather-shaders.md
      fragmentShader: SNOW_FRAG,   // See references/weather-shaders.md
      transparent: true,
      depthWrite: false,
    });

    this.mesh = new THREE.Points(geometry, this.material);
    this.mesh.frustumCulled = false;
    this.scene.add(this.mesh);
  }

  update(time, wind, cameraPos) {
    this.material.uniforms.time.value = time;
    this.material.uniforms.windForce.value.copy(wind.force);
    this.material.uniforms.cameraPos.value.copy(cameraPos);
    this.material.uniforms.intensity.value = this.intensity;
    this.mesh.position.x = cameraPos.x;
    this.mesh.position.z = cameraPos.z;
  }

  dispose() {
    this.scene.remove(this.mesh);
    this.mesh.geometry.dispose();
    this.material.dispose();
  }
}

Splash / Impact System

Ground splashes when rain hits surfaces — instanced ring sprites at impact points.

class SplashSystem {
  constructor(scene, maxSplashes = 500) {
    this.scene = scene;
    this.max = maxSplashes;
    const geo = new THREE.PlaneGeometry(0.15, 0.15);
    geo.rotateX(-Math.PI / 2);

    this.material = new THREE.ShaderMaterial({
      uniforms: { time: { value: 0 } },
      vertexShader: SPLASH_VERT,
      fragmentShader: SPLASH_FRAG,
      transparent: true,
      depthWrite: false,
    });

    this.mesh = new THREE.InstancedMesh(geo, this.material, maxSplashes);
    this.mesh.frustumCulled = false;
    this.lifetimes = new Float32Array(maxSplashes);
    this.nextIdx = 0;
    this.scene.add(this.mesh);
  }

  spawn(position) {
    const dummy = new THREE.Object3D();
    dummy.position.copy(position);
    dummy.position.y += 0.01;
    dummy.scale.setScalar(0.5 + Math.random() * 0.5);
    dummy.updateMatrix();
    this.mesh.setMatrixAt(this.nextIdx, dummy.matrix);
    this.lifetimes[this.nextIdx] = 1.0;
    this.mesh.instanceMatrix.needsUpdate = true;
    this.nextIdx = (this.nextIdx + 1) % this.max;
  }

  update(dt) {
    for (let i = 0; i < this.max; i++) {
      if (this.lifetimes[i] > 0) {
        this.lifetimes[i] -= dt * 4; // ~0.25s lifetime
      }
    }
    this.material.uniforms.time.value += dt;
  }
}

Fog & Atmosphere

Exponential Fog (WebGL — built-in)

function applyFog(scene, density = 0.01, color = 0xcccccc) {
  scene.fog = new THREE.FogExp2(color, density);
  scene.background = new THREE.Color(color);
}

Ground Fog (Shader-based)

Height-attenuated fog that pools in valleys. Applied as a post-process or scene material.

function createGroundFog(size = 200, maxHeight = 8) {
  const geo = new THREE.PlaneGeometry(size, size, 1, 1);
  geo.rotateX(-Math.PI / 2);

  const material = new THREE.ShaderMaterial({
    uniforms: {
      time:      { value: 0 },
      fogColor:  { value: new THREE.Color(0xdddddd) },
      maxHeight: { value: maxHeight },
      density:   { value: 0.8 },
      windDir:   { value: new THREE.Vector2(1, 0) },
      windSpeed: { value: 2.0 },
    },
    vertexShader: `
      varying vec3 vWorldPos;
      varying vec2 vUv;
      void main() {
        vWorldPos = (modelMatrix * vec4(position, 1.0)).xyz;
        vUv = uv;
        gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
      }
    `,
    fragmentShader: GROUND_FOG_FRAG, // See references/weather-shaders.md
    transparent: true,
    depthWrite: false,
    side: THREE.DoubleSide,
  });

  const mesh = new THREE.Mesh(geo, material);
  mesh.position.y = maxHeight * 0.5;
  mesh.renderOrder = 999;
  return mesh;
}

Dust / Sandstorm

Dense particle field with color tinting and reduced visibility.

function createDustStorm(scene, options = {}) {
  const count = options.count ?? 30000;
  const color = options.color ?? new THREE.Color(0xc4a060);
  const positions = new Float32Array(count * 3);
  const seeds = new Float32Array(count);

  for (let i = 0; i < count; i++) {
    positions[i * 3]     = (Math.random() - 0.5) * 80;
    positions[i * 3 + 1] = Math.random() * 15;
    positions[i * 3 + 2] = (Math.random() - 0.5) * 80;
    seeds[i] = Math.random();
  }

  const geo = new THREE.BufferGeometry();
  geo.setAttribute('position', new THREE.BufferAttribute(positions, 3));
  geo.setAttribute('aSeed', new THREE.BufferAttribute(seeds, 1));

  const material = new THREE.ShaderMaterial({
    uniforms: {
      time:      { value: 0 },
      dustColor: { value: color },
      windForce: { value: new THREE.Vector3(8, 0, 2) },
      opacity:   { value: 0.4 },
      pointSize: { value: 4.0 },
    },
    vertexShader: DUST_VERT,
    fragmentShader: DUST_FRAG,
    transparent: true,
    depthWrite: false,
    blending: THREE.NormalBlending,
  });

  const mesh = new THREE.Points(geo, material);
  mesh.frustumCulled = false;
  scene.add(mesh);
  return { mesh, material };
}

Lightning System

Procedural branching lightning bolts with flash illumination.

class LightningSystem {
  constructor(scene) {
    this.scene = scene;
    this.bolts = [];
    this.flashLight = new THREE.PointLight(0xccccff, 0, 500);
    this.flashLight.position.set(0, 80, 0);
    this.scene.add(this.flashLight);
    this._nextStrike = 2 + Math.random() * 8;
    this._flashDecay = 0;
  }

  // Generate a branching bolt path
  generateBolt(start, end, generations = 5, jitter = 15) {
    if (generations <= 0) return [start, end];

    const mid = start.clone().lerp(end, 0.4 + Math.random() * 0.2);
    const perpX = (Math.random() - 0.5) * jitter;
    const perpZ = (Math.random() - 0.5) * jitter;
    mid.x += perpX;
    mid.z += perpZ;

    const left = this.generateBolt(start, mid, generations - 1, jitter * 0.6);
    const right = this.generateBolt(mid, end, generations - 1, jitter * 0.6);

    // Branch: 30% chance at each midpoint
    let branch = [];
    if (Math.random() < 0.3 && generations > 2) {
      const branchEnd = mid.clone().add(
        new THREE.Vector3((Math.random() - 0.5) * jitter * 2, -jitter, (Math.random() - 0.5) * jitter * 2)
      );
      branch = this.generateBolt(mid, branchEnd, generations - 2, jitter * 0.4);
    }

    return [...left, ...right.slice(1), ...branch];
  }

  _createBoltMesh(points) {
    const positions = new Float32Array(points.length * 3);
    for (let i = 0; i < points.length; i++) {
      positions[i * 3]     = points[i].x;
      positions[i * 3 + 1] = points[i].y;
      positions[i * 3 + 2] = points[i].z;
    }
    const geo = new THREE.BufferGeometry();
    geo.setAttribute('position', new THREE.BufferAttribute(positions, 3));

    const material = new THREE.LineBasicMaterial({
      color: 0xeeeeff,
      transparent: true,
      opacity: 1.0,
      linewidth: 2,
    });

    // Core bolt
    const line = new THREE.Line(geo, material);
    // Glow (wider, transparent)
    const glowMat = new THREE.LineBasicMaterial({
      color: 0x8888ff,
      transparent: true,
      opacity: 0.4,
      linewidth: 1,
    });
    const glow = new THREE.Line(geo.clone(), glowMat);
    glow.scale.setScalar(1.02);

    const group = new THREE.Group();
    group.add(line);
    group.add(glow);
    return { group, material, glowMat, life: 0.3 };
  }

  strike(origin, groundY = 0) {
    const start = origin ?? new THREE.Vector3(
      (Math.random() - 0.5) * 100, 70 + Math.random() * 30, (Math.random() - 0.5) * 100
    );
    const end = new THREE.Vector3(start.x + (Math.random() - 0.5) * 20, groundY, start.z + (Math.random() - 0.5) * 20);

    const points = this.generateBolt(start, end, 6, 12);
    const bolt = this._createBoltMesh(points);
    this.scene.add(bolt.group);
    this.bolts.push(bolt);

    // Flash
    this.flashLight.position.copy(start);
    this.flashLight.intensity = 15;
    this._flashDecay = 0.15;
  }

  update(dt, stormIntensity = 0.5) {
    // Auto-strike timing
    this._nextStrike -= dt;
    if (this._nextStrike <= 0 && stormIntensity > 0.3) {
      this.strike();
      this._nextStrike = (1 / stormIntensity) * (3 + Math.random() * 7);
    }

    // Fade bolts
    for (let i = this.bolts.length - 1; i >= 0; i--) {
      const bolt = this.bolts[i];
      bolt.life -= dt;
      const alpha = Math.max(bolt.life / 0.3, 0);
      bolt.material.opacity = alpha;
      bolt.glowMat.opacity = alpha * 0.4;
      if (bolt.life <= 0) {
        this.scene.remove(bolt.group);
        bolt.group.traverse(c => { if (c.geometry) c.geometry.dispose(); });
        this.bolts.splice(i, 1);
      }
    }

    // Flash decay
    if (this._flashDecay > 0) {
      this._flashDecay -= dt;
      this.flashLight.intensity *= 0.85;
    } else {
      this.flashLight.intensity = 0;
    }
  }

  dispose() {
    for (const bolt of this.bolts) {
      this.scene.remove(bolt.group);
    }
    this.scene.remove(this.flashLight);
  }
}

Optical Effects

Rainbow

Arc rendered as a screen-space shader overlay or a mesh arc in world space.

function createRainbow(scene, sunDir) {
  const geo = new THREE.TorusGeometry(120, 3, 16, 64, Math.PI);
  const material = new THREE.ShaderMaterial({
    uniforms: {
      opacity: { value: 0.25 },
    },
    vertexShader: `
      varying vec2 vUv;
      void main() {
        vUv = uv;
        gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
      }
    `,
    fragmentShader: `
      varying vec2 vUv;
      uniform float opacity;
      void main() {
        float t = vUv.x;
        // ROYGBIV spectrum across the arc width
        vec3 col;
        if (t < 0.14)      col = vec3(1.0, 0.0, 0.0);
        else if (t < 0.28) col = vec3(1.0, 0.5, 0.0);
        else if (t < 0.42) col = vec3(1.0, 1.0, 0.0);
        else if (t < 0.57) col = vec3(0.0, 0.8, 0.0);
        else if (t < 0.71) col = vec3(0.0, 0.4, 1.0);
        else if (t < 0.85) col = vec3(0.3, 0.0, 0.8);
        else               col = vec3(0.5, 0.0, 0.5);
        // Soft edges
        float edge = smoothstep(0.0, 0.05, t) * smoothstep(1.0, 0.95, t);
        gl_FragColor = vec4(col, opacity * edge);
      }
    `,
    transparent: true,
    depthWrite: false,
    side: THREE.DoubleSide,
  });

  const rainbow = new THREE.Mesh(geo, material);
  // Position opposite to sun direction
  rainbow.position.set(-sunDir.x * 80, 20, -sunDir.z * 80);
  rainbow.rotation.z = Math.PI;
  rainbow.rotation.y = Math.atan2(-sunDir.x, -sunDir.z);
  scene.add(rainbow);
  return rainbow;
}

Aurora Borealis

Undulating curtain of colored light using a displaced vertical plane.

function createAurora(scene) {
  const geo = new THREE.PlaneGeometry(300, 40, 128, 16);
  const material = new THREE.ShaderMaterial({
    uniforms: {
      time: { value: 0 },
      color1: { value: new THREE.Color(0x00ff88) },
      color2: { value: new THREE.Color(0x4400ff) },
      color3: { value: new THREE.Color(0xff0066) },
    },
    vertexShader: AURORA_VERT,     // See references/weather-shaders.md
    fragmentShader: AURORA_FRAG,   // See references/weather-shaders.md
    transparent: true,
    depthWrite: false,
    side: THREE.DoubleSide,
    blending: THREE.AdditiveBlending,
  });

  const mesh = new THREE.Mesh(geo, material);
  mesh.position.set(0, 60, -100);
  mesh.rotation.x = -0.3;
  scene.add(mesh);
  return { mesh, material };
}

Weather State Machine

Smooth transitions between weather states by interpolating all subsystem parameters.

class WeatherController {
  constructor(scene, camera, wind) {
    this.scene = scene;
    this.camera = camera;
    this.wind = wind;
    this.rain = null;
    this.snow = null;
    this.lightning = null;
    this.fog = null;
    this.currentState = 'clear';
    this.targetState = 'clear';
    this.transition = 1.0;     // 1 = fully arrived at target
    this.transitionSpeed = 0.3; // 0→1 over ~3 seconds
  }

  setState(stateName) {
    if (stateName === this.currentState && this.transition >= 1) return;
    this.currentState = this.targetState;
    this.targetState = stateName;
    this.transition = 0;
  }

  update(dt) {
    this.wind.update(dt);

    if (this.transition < 1) {
      this.transition = Math.min(this.transition + dt * this.transitionSpeed, 1);
    }

    const t = this.transition;
    const from = WEATHER_STATES[this.currentState];
    const to = WEATHER_STATES[this.targetState];

    // Interpolate parameters
    const rainIntensity = lerp(from.rain, to.rain, t);
    const snowIntensity = lerp(from.snow, to.snow, t);
    const fogDensity = lerp(from.fogDensity, to.fogDensity, t);
    const stormIntensity = lerp(from.lightning, to.lightning, t);
    const skyDarkness = lerp(from.skyDarkness, to.skyDarkness, t);
    const windMult = lerp(from.windMultiplier, to.windMultiplier, t);

    this.wind.baseSpeed = 5 * windMult;

    // Update subsystems
    if (this.rain) {
      this.rain.intensity = rainIntensity;
      this.rain.update(performance.now() * 0.001, this.wind, this.camera.position);
    }
    if (this.snow) {
      this.snow.intensity = snowIntensity;
      this.snow.update(performance.now() * 0.001, this.wind, this.camera.position);
    }
    if (this.lightning) {
      this.lightning.update(dt, stormIntensity);
    }

    // Fog
    if (this.scene.fog) {
      this.scene.fog.density = fogDensity;
    }

    // Sky darkening
    const skyColor = new THREE.Color(0x87ceeb).lerp(new THREE.Color(0x333340), skyDarkness);
    if (this.scene.background instanceof THREE.Color) {
      this.scene.background.copy(skyColor);
    }
  }
}

function lerp(a, b, t) { return a + (b - a) * t; }

const WEATHER_STATES = {
  clear:      { rain: 0, snow: 0, fogDensity: 0.0005, lightning: 0, skyDarkness: 0, windMultiplier: 0.5 },
  cloudy:     { rain: 0, snow: 0, fogDensity: 0.002, lightning: 0, skyDarkness: 0.2, windMultiplier: 0.8 },
  drizzle:    { rain: 0.3, snow: 0, fogDensity: 0.004, lightning: 0, skyDarkness: 0.3, windMultiplier: 0.7 },
  rain:       { rain: 0.7, snow: 0, fogDensity: 0.006, lightning: 0.1, skyDarkness: 0.5, windMultiplier: 1.2 },
  heavyRain:  { rain: 1.0, snow: 0, fogDensity: 0.01, lightning: 0.3, skyDarkness: 0.7, windMultiplier: 1.8 },
  storm:      { rain: 1.0, snow: 0, fogDensity: 0.015, lightning: 0.8, skyDarkness: 0.85, windMultiplier: 2.5 },
  lightSnow:  { rain: 0, snow: 0.3, fogDensity: 0.003, lightning: 0, skyDarkness: 0.15, windMultiplier: 0.6 },
  snow:       { rain: 0, snow: 0.7, fogDensity: 0.006, lightning: 0, skyDarkness: 0.3, windMultiplier: 1.0 },
  blizzard:   { rain: 0, snow: 1.0, fogDensity: 0.025, lightning: 0, skyDarkness: 0.6, windMultiplier: 3.0 },
  fog:        { rain: 0, snow: 0, fogDensity: 0.03, lightning: 0, skyDarkness: 0.25, windMultiplier: 0.2 },
  sandstorm:  { rain: 0, snow: 0, fogDensity: 0.02, lightning: 0.1, skyDarkness: 0.5, windMultiplier: 3.0 },
};

Camera Post-Processing

Wet Lens Effect

Droplets on screen when raining, applied as a fullscreen overlay.

function createWetLensOverlay(intensity = 0.5) {
  return new THREE.ShaderMaterial({
    uniforms: {
      time:      { value: 0 },
      intensity: { value: intensity },
      tScene:    { value: null },
    },
    vertexShader: FULLSCREEN_VERT,
    fragmentShader: WET_LENS_FRAG, // See references/weather-shaders.md
    transparent: true,
  });
}

Frost Overlay

Progressive frost creep from screen edges during cold/blizzard conditions.

function createFrostOverlay(intensity = 0.0) {
  return new THREE.ShaderMaterial({
    uniforms: {
      intensity: { value: intensity }, // 0=clear, 1=fully frosted
      time:      { value: 0 },
    },
    vertexShader: FULLSCREEN_VERT,
    fragmentShader: FROST_FRAG,
    transparent: true,
  });
}

Complete Scene Assembly

async function init() {
  const canvas = document.querySelector('#canvas');
  const { renderer } = await createRenderer(canvas);

  const scene = new THREE.Scene();
  scene.background = new THREE.Color(0x87ceeb);
  scene.fog = new THREE.FogExp2(0x87ceeb, 0.001);

  const camera = new THREE.PerspectiveCamera(60, innerWidth / innerHeight, 0.5, 500);
  camera.position.set(0, 5, 20);

  const { OrbitControls } = await import('three/addons/controls/OrbitControls.js');
  const controls = new OrbitControls(camera, renderer.domElement);
  controls.enableDamping = true;

  // Ground
  const ground = new THREE.Mesh(
    new THREE.PlaneGeometry(200, 200),
    new THREE.MeshStandardMaterial({ color: 0x4a7c3f, roughness: 0.9 })
  );
  ground.rotation.x = -Math.PI / 2;
  ground.receiveShadow = true;
  scene.add(ground);

  // Lighting
  const sun = new THREE.DirectionalLight(0xfff4e5, 1.2);
  sun.position.set(30, 40, 20);
  scene.add(sun);
  scene.add(new THREE.HemisphereLight(0x87ceeb, 0x4a7c3f, 0.5));

  // Weather
  const wind = new WindSystem();
  const weather = new WeatherController(scene, camera, wind);
  weather.rain = new RainSystem(scene);
  weather.snow = new SnowSystem(scene);
  weather.lightning = new LightningSystem(scene);

  // Start with rain
  weather.setState('rain');

  const clock = new THREE.Clock();
  renderer.setAnimationLoop(() => {
    weather.update(clock.getDelta());
    controls.update();
    renderer.render(scene, camera);
  });

  window.addEventListener('resize', () => {
    camera.aspect = innerWidth / innerHeight;
    camera.updateProjectionMatrix();
    renderer.setSize(innerWidth, innerHeight);
  });
}

init();

Performance Guidelines

Total budget: All weather combined stays under 5 draw calls and ~100K particles.

Key rules:

• All animation in vertex shaders via uniforms (time, wind). Zero per-frame JS particle loops.
• frustumCulled = false on all particle meshes — spawn box follows camera.
• Recenter spawn box on camera each frame so particles always surround the viewer.
• depthWrite: false on all transparent weather effects to avoid sorting artifacts.

Common Pitfalls

1. Rain falls through roof: Precipitation has no collision. For indoor scenes, cull particles below a Y threshold or use a height map mask uniform.
2. Snow accumulation: Not simulated per-particle. Use a ground plane shader that blends white coverage based on snowIntensity * time, or a decal system.
3. Lightning too regular: Randomize interval per stormIntensity. Real lightning is clustered — bursts of 2–3 strikes then silence.
4. Fog and sky mismatch: Always set scene.fog.color = scene.background. Mismatched colors create visible hard edges at the distance limit.
5. Weather pops on/off: Always use the state machine transition. Interpolating intensity from 0→target over 2–3 seconds looks natural.

References

• references/weather-shaders.md — Complete GLSL vertex/fragment shaders for rain, snow, dust, ground fog, aurora, wet lens, frost, and WGSL compute particles.
• references/weather-types.md — Detailed profiles for 12 weather states with parameter tables, artistic direction, environmental effects, and combination rules.