Edit in JSFiddle

(function($){
    var $snow = $('<div id="snowBox" />').css({'position': 'absolute', 'top': '-50px'}).html('&#10052;');
	var h 	= $(document).height();
	var w	= $(document).width();
    var t = setInterval( function(){
		var startPositionLeft 	= Math.random() * w - 100,
		 	startOpacity		= 0.5 + Math.random(),
			sizeFlake			= 10 + Math.random() * 30,
			endPositionTop		= h - 40,
			endPositionLeft		= startPositionLeft - 100 + Math.random() * 500,
			durationFall		= h * 10 + Math.random() * 4000;
			$snow.clone().appendTo('body').css({
				'left': startPositionLeft,
				'opacity': startOpacity,
				'font-size': sizeFlake,
				'color': '#ffffff'
			}).animate({
				'top': endPositionTop,
				'left': endPositionLeft,
				'opacity': 0.2
			},durationFall,'linear',function(){
				$(this).remove();
			});					
	}, 1000);
})(jQuery);

body{background:#222222 url(http://www.17sucai.com/upload/32215/2013-10-23/eb32c1fb998feb6576dab0cfac50f32a.jpg) no-repeat center center;background-size:cover; color:#ccc;height:750px;overflow-x:hidden;}