Special documents effects Autumn Leaves

I use it to display snowflakes, but only on the banner of my webpages. Instead of calculating the window heigth, I fixed the value to 80. So far perfect !
Only problem is that when I scroll down the page, the banner is not visible anymore (OK) but it is still snowing on the first lines of the webpage.

I use another dhtml script for textscrolling and I have no problem of display when scrolling down the page.

How to modify the script so that it only applies to a visible div section ?

This is my website if you want to see it in action..just scroll down the mainpage and you will see the snow always on the top of the page :