Flocon de neige qui tombent mais pas jusqu'en bas. Pourquoi ?

var ns4b=(document.layers)?1:0,b4up=(document.all)?1:0,doc_width=1000,doc_height=1200,dx,xp,yp,am,stx,sty,snowobj,i;
	if(ns4b){
		doc_width=self.innerWidth;
		doc_height=self.innerHeight
	}else{
		if(b4up){
			doc_width=document.body.clientWidth;
			doc_height=document.body.clientHeight
		}else{
			b4up=1
		}
	}
dx=new Array();
xp=new Array();
yp=new Array();
am=new Array();
stx=new Array();
sty=new Array();
snowobj=new Array();
for(i=0;i<no;++i){
	dx[i]=0;xp[i]=Math.random()*(doc_width-50);
	yp[i]=Math.random()*doc_height;am[i]=Math.random()*20;
	stx[i]=0.02+Math.random()/10;sty[i]=0.7+Math.random();
	if(ns4b){
		document.write('<LAYER NAME="flake'+i+'" LEFT="15" TOP="15" VISIBILITY="show"><IMG SRC="'+snowflake+'" BORDER=0></LAYER>')
	}else{
		if(b4up){
			document.write('<DIV ID="flake'+i+'" STYLE="position:absolute; z-index:'+i+'; visibility:visible; top:15px; left:15px;"><IMG SRC="'+snowflake+'" BORDER=0></DIV>');
			snowobj[i]=eval(document.getElementById("flake"+i).style)}
		}
	}
	
function snowNS(){
	 for(i=0;i<no;++i){yp[i]+=sty[i];if(yp[i]>doc_height-50){xp[i]=Math.random()*(doc_width-am[i]-30);yp[i]=0;stx[i]=0.02+Math.random()/10;sty[i]=0.7+Math.random()}dx[i]+=stx[i];
	 document.layers["flake"+i].top=yp[i];document.layers["flake"+i].left=xp[i]+am[i]*Math.sin(dx[i])}setTimeout("snowNS()",speed)
}
function snowDocument(){
	for(i=0;i<no;++i){
		yp[i]+=sty[i];if(yp[i]>doc_height-50){xp[i]=Math.random()*(doc_width-am[i]-30);yp[i]=0;
		stx[i]=0.02+Math.random()/10;sty[i]=0.7+Math.random()
	}
	dx[i]+=stx[i];
	snowobj[i].top=yp[i];
	snowobj[i].left=xp[i]+am[i]*Math.sin(dx[i])}setTimeout("snowDocument()",speed)
}
if(ns4b){
		snowNS();
	}else{
		if(b4up){snowDocument()
	}
};