SOURCE

-(function(){

	var Grewer = {
		init:function(option){
			option = option || {};

			this.speed = option.speed || 5;//雪花下降速度

			//获取窗口宽度	
			this.getWindowSize();


			this.main = document.getElementById('snow');


			this.main.style.width = this.winWidth + 'px';
			this.main.style.height = this.winHeight + 'px'

			document.body.style.margin = 0;


			this.main.style.backgroundColor = '#79c6e8';
			//背景色

			this.arr = [];

			this.run(option);
		},
		getWindowSize:function(){
			if(window.innerWidth){  
				this.winWidth=window.innerWidth;
			}else if((document.body)&&(document.body.clientWidth)){   
				this.winWidth=document.body.clientWidth;
			}

				if(window.innerHeight){
					this.winHeight=window.innerHeight;
				}else if((document.body)&&(document.body.clientHeight)){
					this.winHeight=document.body.clientHeight;
				}
				
			},
			render:function(num){
				this.Num = (num * Math.random())|0;
				//雪花数
				while(this.Num--){
					var RS = {
						x:(Math.random()*this.winWidth)|0,
						y:0,
						stepX:(Math.random()*5-2)|0,
						stepY:((Math.random()*this.speed)|0)+this.speed
					}

					var img = document.createElement('div');
					img.style.width = '20px';
					img.style.height = '20px';

					//img.style.background = 'url(https://files.cnblogs.com/files/Grewer/snow.gif) no-repeat -9px -3px';
					//img.style.backgroundSize = '36px 27px';

					img.innerHTML = '*';
					img.style.color = '#fff';

					img.style.position = 'absolute';

					img.style.left = RS.x + 'px';
					img.style.top = (RS.y-20)+'px';
					this.main.appendChild(img);

					RS.el = img;


					this.arr.push(RS);

				}
			},
			update:function(){
				var i = this.arr.length;
				while(i--){
					var list = this.arr[i],
					obj = list.el;

					list.x += list.stepX;
					obj.style.left = list.x + 'px';

					list.y += list.stepY;
					obj.style.top = list.y + 'px';

					if(this.check(list)){
						this.arr.splice(i,1);
					}
				}
				
			},
			removeElement:function (_element){
				var _parentElement = _element.parentNode;
				if(_parentElement){
					_parentElement.removeChild(_element);
				}
			},
			check:function(list){
				if(list.x-20 <= 0){
					this.removeElement(list.el)
					return true
				}

				if(list.x+20 >= this.winWidth){
					this.removeElement(list.el)
					return true
				}

				if(list.y+20 > this.winHeight){
					this.removeElement(list.el)
					return true;
				}

				return false;
			},
			run:function(option){
				setInterval(function(){
					snow.update();
				}
				,100);
				setInterval(function(){
					snow.render(option.num||10)
				},option.snowFlash||1000)
			}

		}


		window.snow = Object.create(Grewer);

})()
		// obj.init({
		// 	'speed':5,
		// 	'num':10,
		// 	'snowFlash':1000
		// });//加入参数

		
	snow.init();

<div id="snow">
	</div>