JQuery window scrolling event?

Question

Try this: http://jsbin.com/axaler/3/edit

$(function(){
  $(window).scroll(function(){
    var aTop = $('.ad').height();
    if($(this).scrollTop()>=aTop){
        alert('header just passed.');
        // instead of alert you can use to show your ad
        // something like $('#footAd').slideup();
    }
  });
});

See jQuery.scroll(). You can bind this to the window element to get your desired event hook.

On scroll, then simply check your scroll position:

$(window).scroll(function() {
  var scrollTop = $(window).scrollTop();
  if ( scrollTop > $(headerElem).offset().top ) { 
    // display add
  }
});