Test object top above window top

$(document).on('scroll',function(){
	var winTop = $(window).scrollTop();
	var $object = $('.some-div');
	$object.each(function(){
		var objTopTrigger = this.offsetTop;
		var objTopTriggerAdj = 400;
		if (objTopTrigger + objTopTriggerAdj < winTop){
			$(this).addClass('some-class');
		}
		if (objTopTrigger + objTopTriggerAdj > winTop){
			$(this).removeClass('some-class');
		}
	});
});