snow.js

function randomPointOnTopOfSphere(radius){
  var theta = Math.PI * Math.random();
  var phi = Math.acos((2 * Math.random() - 1) * Math.PI/2);
  var x = radius * Math.sin(phi) * Math.cos(theta);
  var y = radius * Math.sin(phi) * Math.sin(theta);
  var z = radius * Math.cos(phi);
  return { x: x, y: y, z: z };
}

function randomPointInsideSphere(radius){
  var theta = 2 * Math.PI * Math.random();
  var phi = Math.acos((2 * Math.random() - 1) * Math.PI/2);
  var u = Math.min(Math.random() * radius * 3, radius);
  var x = u * Math.sin(phi) * Math.cos(theta);
  var y = u * Math.sin(phi) * Math.sin(theta);
  var z = u * Math.cos(phi);
  return { x: x, y: y, z: z };
}

function create_snow(particle_size, particle_count, radius) {
  // create the particle variables
  particle_size = particle_size || 1.5;
  particle_count = particle_count ||500;
  radius = radius || 50;

  var particles = new THREE.Geometry();
  var pMaterial = new THREE.PointsMaterial({
        color: 0xFFFFFF,
        size: particle_size,
        map: THREE.ImageUtils.loadTexture( "snowflake.png" ),
        blending: THREE.AdditiveBlending,
        transparent: true
      });
  pMaterial.depthWrite = false;

  // Creating individual particles
  for (var p = 0; p < particle_count; p++) {
    // create a particle with random position inside sphere
    var position = randomPointInsideSphere(radius - particle_size);
        particle = new THREE.Vector3(position.x, position.y, position.z);
    // add it to the geometry
    particles.vertices.push(particle);
  }

  // create the particle system
  var particleSystem = new THREE.Points( particles, pMaterial);

  return {
    particle_size: particle_size,
    particle_count: particle_count,
    radius: radius - particle_size,
    particleSystem: particleSystem
  };
}

function render_snow(snow) {
  var pCount = 0;
  while(pCount < snow.particle_count) {
    var particle = snow.particleSystem.geometry.vertices[pCount];

    square_d = (particle.x*particle.x + particle.y*particle.y + particle.z*particle.z);
    if(square_d > snow.radius*snow.radius) {
      var new_point = randomPointOnTopOfSphere(snow.radius);
      particle.set(new_point.x, new_point.y, new_point.z);
    }

    particle.y -= .1;
    pCount++;
  }

  snow.particleSystem.geometry.verticesNeedUpdate = true;
}