Fading away an element when scrolling down and fading it back when scrolling up in WordPress. http://sridharkatakam.com/fade-away-element-scrolling-fade-back-scrolling-wordpress/

functions.php

add_action( 'wp_enqueue_scripts', 'sk_enqueue_scripts' );
function sk_enqueue_scripts() {

	if ( !is_home() || !is_front_page() )
		return;

	wp_enqueue_script( 'scrollTo', get_stylesheet_directory_uri() . '/js/jquery.scrollTo.min.js', array( 'jquery' ), '1.4.5-beta', true );
	wp_enqueue_script( 'home', get_stylesheet_directory_uri() . '/js/home.js', array( 'scrollTo' ), '', true );

}

home.js

jQuery(function( $ ){

	function fade_home_top() {
		if ( $(window).width() > 800 ) {
        window_scroll = $(this).scrollTop();
	   		$("#home_bkgrd_image").css({
				  'opacity' : 1-(window_scroll/300)
	    	});
		}
	}
	$(window).scroll(function() { fade_home_top(); });

});