Fade in element on scroll down using css

Demo Fiddle

Do you want something like this ?

   $(window).scroll(function () {

        /* Check the location of each desired element */
        $('.article').each(function (i) {

            var bottom_of_object = $(this).position().top + $(this).outerHeight();
            var bottom_of_window = $(window).scrollTop() + $(window).height();

            /* If the object is completely visible in the window, fade it it */
            if (bottom_of_window > bottom_of_object) {

                $(this).animate({
                    'opacity': '1'
                }, 500);

            }

        });

    });

Mohammeds answer does not take into account any absolutely positioned images... we should use offset instead of position

$(window).scroll(function () {

    /* Check the location of each desired element */
    $('.article').each(function (i) {

        var bottom_of_object = $(this).offset().top + $(this).outerHeight();
        var bottom_of_window = $(window).scrollTop() + $(window).height();

        /* If the object is completely visible in the window, fade it it */
        if (bottom_of_window > bottom_of_object) {

            $(this).animate({
                'opacity': '1'
            }, 500);

         }
     });
});

Because I read it in the comments:

If you want the object to fade in when 1/3 of the object is visible, then use this code:

jQuery:

$(document).ready(function() {

    /* Every time the window is scrolled ... */
    $(window).scroll( function(){

        /* Check the location of each desired element */
        $('.hideme').each( function(i){

            var bottom_of_object = $(this).position().top + ($(this).outerHeight())/3;
            var bottom_of_window = $(window).scrollTop() + $(window).height();

            /* If the object is completely visible in the window, fade it it */
            if( bottom_of_window > bottom_of_object ){

                $(this).animate({'opacity':'1'},500);

            }

        });

    });

});

Only line I changed, is this:

var bottom_of_object = $(this).position().top + ($(this).outerHeight())/3;

By changing /3 to e.g. /4 you can let it fade in, when 1/4 of the object is visible.