snow.js

//  This example was created by Jens Anders Bakke

var game = new Phaser.Game(800, 600, Phaser.AUTO, 'phaser-example', { preload: preload, create: create });

function preload() {

    game.load.spritesheet('snowflakes', 'assets/sprites/snowflakes.png', 17, 17);
    game.load.spritesheet('snowflakes_large', 'assets/sprites/snowflakes_large.png', 64, 64);

}

var max = 0;
var front_emitter;
var mid_emitter;
var back_emitter;
var update_interval = 4 * 60;
var i = 0;

function create() {

    back_emitter = game.add.emitter(game.world.centerX, -32, 600);
    back_emitter.makeParticles('snowflakes', [0, 1, 2, 3, 4, 5]);
    back_emitter.maxParticleScale = 0.6;
    back_emitter.minParticleScale = 0.2;
    back_emitter.setYSpeed(20, 100);
    back_emitter.gravity = 0;
    back_emitter.width = game.world.width * 1.5;
    back_emitter.minRotation = 0;
    back_emitter.maxRotation = 40;


    mid_emitter = game.add.emitter(game.world.centerX, -32, 250);
    mid_emitter.makeParticles('snowflakes', [0, 1, 2, 3, 4, 5]);
    mid_emitter.maxParticleScale = 1.2;
    mid_emitter.minParticleScale = 0.8;
    mid_emitter.setYSpeed(50, 150);
    mid_emitter.gravity = 0;
    mid_emitter.width = game.world.width * 1.5;
    mid_emitter.minRotation = 0;
    mid_emitter.maxRotation = 40;


    front_emitter = game.add.emitter(game.world.centerX, -32, 50);
    front_emitter.makeParticles('snowflakes_large', [0, 1, 2, 3, 4, 5]);
    front_emitter.maxParticleScale = 1;
    front_emitter.minParticleScale = 0.5;
    front_emitter.setYSpeed(100, 200);
    front_emitter.gravity = 0;
    front_emitter.width = game.world.width * 1.5;
    front_emitter.minRotation = 0;
    front_emitter.maxRotation = 40;

    changeWindDirection();

    back_emitter.start(false, 14000, 20);
    mid_emitter.start(false, 12000, 40);
    front_emitter.start(false, 6000, 1000);

}

function update() {

    i++;

    if (i === update_interval)
    {
        changeWindDirection();
        update_interval = Math.floor(Math.random() * 20) * 60; // 0 - 20sec @ 60fps
        i = 0;
    }

}

function changeWindDirection() {

    var multi = Math.floor((max + 200) / 4),
        frag = (Math.floor(Math.random() * 100) - multi);
    max = max + frag;

    if (max > 200) max = 150;
    if (max < -200) max = -150;

    setXSpeed(back_emitter, max);
    setXSpeed(mid_emitter, max);
    setXSpeed(front_emitter, max);

}

function setXSpeed(emitter, max) {

    emitter.setXSpeed(max - 20, max);
    emitter.forEachAlive(setParticleXSpeed, this, max);

}

function setParticleXSpeed(particle, max) {

    particle.body.velocity.x = max - Math.floor(Math.random() * 30);

}