source: other-projects/nz-flag-design/trunk/render-3d/weather/rain.js

function rain() {
    
    this.isInit = false;
    this.MASS = 0.9;
    
    this.init = function() {
    
        // modify lighting
        this.HSL = light.color.getHSL();
        light.color.setHSL(this.HSL.h, this.HSL.s, 0.2);
    
        if(this.isInit === true) return;
    
        // create the particle variables
        this.particleCount = 8000,
        this.particles = new THREE.Geometry(),
        this.pMaterial = new THREE.ParticleBasicMaterial({
            color: 0xFFFFFF,
            size: 15,
            map: THREE.ImageUtils.loadTexture(
                "./images/raindrop.png"
            ),
            blending: THREE.AdditiveBlending,
            transparent: true,
            opacity: 1
        });

        // now create the individual particles
        for (var p = 0; p < this.particleCount; p++) {

            // create a particle with random
            // position values, -250 -> 250
            var pX = (Math.random() * 10000) - 5000,
                pY = Math.random() * window.innerHeight * 6 - window.innerHeight,
                pZ = (Math.random() * 10000) - 5000;
                this.particle = new THREE.Vector3(pX, pY, pZ);  

            // create a velocity vector
            this.particle.velocity = new THREE.Vector3(
                (windForce.x * this.MASS),  // x
                -Math.random() * this.MASS, // y
                (windForce.z * this.MASS)   // z
            ); 

            // add it to the geometry
            this.particles.vertices.push(this.particle);
        }

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

        this.particleSystem.sortParticles = true;
        this.isInit = true;
    }

    this.system = function(){
        return this.particleSystem;
    }

    this.update = function() {

        // add some rotation to the system
        //particleSystem.rotation.y += 0.01;
        
        var pCount = this.particleCount;
        while(pCount--) {
            // get the particle
            this.particle = this.particles.vertices[pCount];
            
            // check if we need to reset
            if(this.particle.y < -10) {
                this.particle.y = (window.innerHeight*6)+(Math.random() * 100);
                this.particle.velocity.y = 0;
                this.particle.x = (Math.random() * 10000) - 5000;
                this.particle.z = (Math.random() * 10000) - 5000;
            }
            
            // update the velocity
            this.particle.velocity.y -= Math.random() * 0.01;
            this.particle.velocity.x = windForce.x * (1-this.MASS);
            this.particle.velocity.z = windForce.z * (1-this.MASS)
            
            // and the position
            this.particle.addVectors( 
                this.particle,
                this.particle.velocity);
        }
        
        // flag to the particle system that we've
        // changed its vertices.
        this.particleSystem.geometry.__dirtyVertices = true;
    }
    
}