下雪效果

displaySnow();

function displaySnow() {
	for (var i:int=0; i<40; i++) {
		var snowFlake:SnowFlake=new SnowFlake(400);
		addChild(snowFlake);

		snowFlake.x=Math.random()*550;
		snowFlake.y=Math.random()*400;

		snowFlake.alpha=0.2+Math.random()*5;

		snowFlake.scaleX=snowFlake.scaleY=0.4+Math.random()*1.5;
	}
}

//===================雪花类SnowFlake.as===================
package {
	import flash.display.MovieClip;
	import flash.events.Event;

	public class SnowFlake extends MovieClip {
		private var radians:Number=0;
		private var speed:Number=0;

		private var maxHeight:Number;

		public function SnowFlake(maxHeight_:Number) {
			speed=0.5*Math.random();//[0.01,0.51)

			maxHeight=maxHeight_;

			this.addEventListener(Event.ENTER_FRAME,snowing);
		}

		private function snowing(e:Event):void {
			radians+=speed;

			//如果此处不用Math.cos()，而用普通的this.x+=Math.random()*2，左右晃动效果会很明显
			this.x+=Math.cos(radians);
			this.y+=2;

			if (this.y>maxHeight) {
				this.y=-20;
			}
		}
	}
}