Stopping the Falling Snow

Im working on a flash movie right now, and for a certain amount of frames I want falling snow on the screen.

I have looked at the falling snow tutorial, and it works perfect for what i want, my only problem is, how do I get it to stop?

Heres the actionscript on my movie clip of the snowflake:

*onClipEvent (load) {
//specifies the size of the movie stage
movieWidth = 590;
movieHeight = 450;

//variables that will modify the falling snow 
i = 1 Math.random()*2; 
k = -Math.PI Math.random()*Math.PI; 

//giving each snowflake unique characteristics 
this._xscale = this._yscale=50 Math.random()*100; 
this._alpha = 75 Math.random()*100; 
this._x = -10 Math.random()*movieWidth; 
this._y = -10 Math.random()*movieHeight; 

}
onClipEvent (enterFrame) {
//putting it all together
rad = (k/180)*Math.PI;
this._x -= Math.cos(rad);
this._y = i;
if (this._y>=movieHeight) {
this._y = -5;
}
if ((this._x>=movieWidth) || (this._x<=0)) {
this._x = -10 Math.random()*movieWidth;
this._y = -5;
}
}
*
Now I want the snow to start falling at frame 500 so I put this script on frame 500:

*for (k=0; k<50; k ) {
duplicateMovieClip(this.snow, “snow” k, k);
} *

Now like i said this works perfect for what I want, the snow starts falling on frame 500, but I want it to stop at frame 800, and it doesnt stop, can anyone help me? I would greatly appreciate it.

-a