<script type="text/javascript">
      /* 控制下雪 */
      function snowFall(snow) {
           /* 可配置属性 */
           snow = snow || {};
           this.maxFlake = snow.maxFlake || 200;   /* 最多片数 */
           this.flakeSize = snow.flakeSize || 10;  /* 雪花形状 */
           this.fallSpeed = snow.fallSpeed || 1;   /* 坠落速度 */
       }
    /* 兼容写法 */
    requestAnimationFrame = window.requestAnimationFrame ||
            window.mozRequestAnimationFrame ||
        window.webkitRequestAnimationFrame ||
        window.msRequestAnimationFrame ||
        window.oRequestAnimationFrame ||
        function(callback) { setTimeout(callback, 1000 / 60); };

    cancelAnimationFrame = window.cancelAnimationFrame ||
            window.mozCancelAnimationFrame ||
        window.webkitCancelAnimationFrame ||
        window.msCancelAnimationFrame ||
        window.oCancelAnimationFrame;
    /* 开始下雪 */
    snowFall.prototype.start = function(){
            /* 创建画布 */
            snowCanvas.apply(this);
            /* 创建雪花形状 */
            createFlakes.apply(this);
            /* 画雪 */
            drawSnow.apply(this)
        }
    /* 创建画布 */
    function snowCanvas() {
            /* 添加Dom结点 */
            var snowcanvas = document.createElement("canvas");
            snowcanvas.id = "snowfall";
            snowcanvas.width = window.innerWidth;
            snowcanvas.height = document.body.clientHeight;
            snowcanvas.setAttribute("style", "position:absolute; top: 0; left: 0; z-index: 1; pointer-events: none;");
            document.getElementsByTagName("body")[0].appendChild(snowcanvas);
            this.canvas = snowcanvas;
            this.ctx = snowcanvas.getContext("2d");
            /* 窗口大小改变的处理 */
            window.onresize = function() {
                 snowcanvas.width = window.innerWidth;
                 /* snowcanvas.height = window.innerHeight */
             }
        }
    /* 雪运动对象 */
    function flakeMove(canvasWidth, canvasHeight, flakeSize, fallSpeed) {
            this.x = Math.floor(Math.random() * canvasWidth);   /* x坐标 */
            this.y = Math.floor(Math.random() * canvasHeight);  /* y坐标 */
            this.size = Math.random() * flakeSize + 2;          /* 形状 */
            this.maxSize = flakeSize;                           /* 最大形状 */
            this.speed = Math.random() * 1 + fallSpeed;         /* 坠落速度 */
            this.fallSpeed = fallSpeed;                         /* 坠落速度 */
            this.velY = this.speed;                             /* Y方向速度 */
            this.velX = 0;                                      /* X方向速度 */
            this.stepSize = Math.random() / 30;                 /* 步长 */
            this.step = 0                                       /* 步数 */
        }
    flakeMove.prototype.update = function() {
            var x = this.x,
                  y = this.y;
          /* 左右摆动(余弦) */
          this.velX *= 0.98;
          if (this.velY <= this.speed) {
                  this.velY = this.speed
              }
          this.velX += Math.cos(this.step += .05) * this.stepSize;

          this.y += this.velY;
          this.x += this.velX;
          /* 飞出边界的处理 */
          if (this.x >= canvas.width || this.x <= 0 || this.y >= canvas.height || this.y <= 0) {
                  this.reset(canvas.width, canvas.height)
              }
        };
    /* 飞出边界-放置最顶端继续坠落 */
    flakeMove.prototype.reset = function(width, height) {
            this.x = Math.floor(Math.random() * width);
            this.y = 0;
            this.size = Math.random() * this.maxSize + 2;
            this.speed = Math.random() * 1 + this.fallSpeed;
            this.velY = this.speed;
            this.velX = 0;
        };
    // 渲染雪花-随机形状（此处可修改雪花颜色！！！）
    flakeMove.prototype.render = function(ctx) {
            var snowFlake = ctx.createRadialGradient(this.x, this.y, 0, this.x, this.y, this.size);
            snowFlake.addColorStop(0, "rgba(255, 255, 255, 0.9)");  /* 此处是雪花颜色，默认是白色 */
            snowFlake.addColorStop(.5, "rgba(255, 255, 255, 0.5)"); /* 若要改为其他颜色，请自行查 */
            snowFlake.addColorStop(1, "rgba(255, 255, 255, 0)");    /* 找16进制的RGB 颜色代码。 */
            ctx.save();
            ctx.fillStyle = snowFlake;
            ctx.beginPath();
            ctx.arc(this.x, this.y, this.size, 0, Math.PI * 2);
            ctx.fill();
            ctx.restore();
         };
     /* 创建雪花-定义形状 */
     function createFlakes() {
         var maxFlake = this.maxFlake,
                 flakes = this.flakes = [],
                 canvas = this.canvas;
         for (var i = 0; i < maxFlake; i++) {
                 flakes.push(new flakeMove(canvas.width, canvas.height, this.flakeSize, this.fallSpeed))
             }
         }
     /* 画雪 */
     function drawSnow() {
             var maxFlake = this.maxFlake,
                  flakes = this.flakes;
          ctx = this.ctx, canvas = this.canvas, that = this;
          /* 清空雪花 */
          ctx.clearRect(0, 0, canvas.width, canvas.height);
          for (var e = 0; e < maxFlake; e++) {
                  flakes[e].update();
                  flakes[e].render(ctx);
              }
          /*  一帧一帧的画 */
          this.loop = requestAnimationFrame(function() {
                  drawSnow.apply(that);
              });
         }
        /* 调用及控制方法 */
        var snow = new snowFall({maxFlake:60});
        snow.start();
    </script>