Edit in JSFiddle

window.requestAnimFrame = (function(){
    return  window.requestAnimationFrame   ||
        window.webkitRequestAnimationFrame ||
        window.mozRequestAnimationFrame    ||
        window.oRequestAnimationFrame      ||
        window.msRequestAnimationFrame     ||
        function(callback){
            window.setTimeout(callback, 1000 / 60);
        };
})();


var canvas = document.querySelector('#canvas-container');
var ctx = canvas.getContext('2d');

var Particle = function(scale, color, vx, vy, gv) {
    this.scale = scale; //大きさ
    this.color = color; //色
    this.vx = vx; //X速度
    this.vy = vy; //Y速度
    this.gv = gv; //重力
    this.position = {   // 位置
        x: 0,
        y: 0
    };
};

Particle.prototype.draw = function() {
    ctx.beginPath();
    ctx.arc(this.position.x, this.position.y, this.scale, 0, 2*Math.PI, false);
    ctx.fillStyle = this.color;
    ctx.fill();
};

Particle.prototype.update = function() {
    this.vy += this.gv;
    this.position.x += this.vx;
    this.position.y += this.vy;

    // 地面の衝突判定
    if (this.position.y > canvas.height - this.scale) {
        this.vy *= -0.6;
        this.vx *= 0.85;
        this.position.y = canvas.height - this.scale;
    }
    this.draw();
};


var density = 100;  //パーティクルの密度
var particles = []; //パーティクルをまとめる配列
var colors = ['#D0A000', '#6DD0A5', '#E084C5'];

for (var i=0; i<density; i++) {
    var color = colors[~~(Math.random()*3)];
    var scale = ~~(Math.random()*5)+3;
    var x = Math.random() * 10 - 5;
    var y = Math.random()* 9 + 4;
    var g = Math.random()* 0.2 + 0.3;
    
    particles[i] = new Particle(scale, color, x, -y, g);
    particles[i].position.x = canvas.width / 2;
    particles[i].position.y = 200;
    
}

loop();



// ループ処理
function loop() {
    requestAnimFrame(loop);
    // 描画をクリアー
    ctx.clearRect(0,0, ctx.canvas.width, ctx.canvas.height);
    for (var i in particles) {
        particles[i].update();
    }
}

<canvas id="canvas-container" width="600" height="480"></canvas>