JS: Fixed Header

// decreases header size when user scrolls down
        avia_header_size();
//check if the browser supports element rotation
    function avia_header_size()
    {
        var win	            = $(window),
            header          = $('.fixed_header #header'),
            //logo			= header.find('.logo img'),
            logo            = $('#header_main .container .logo img, #header_main .container .logo a'),
            elements        = $('#header_main .container, .main_menu ul:first-child > li > a:not(.avia_mega_div a)'),
            el_height       = $(elements).filter(':first').height(),
            isMobile        = 'ontouchstart' in document.documentElement,
            scroll_top		= $('#scroll-top-link'),
            set_height      = function()
            {
                var st = win.scrollTop(), newH = 0;

                if(st < el_height/2)
                {
                    newH = el_height - st;
                    header.removeClass('header-scrolled');
                }
                else
                {
                    newH = el_height/2;
                    header.addClass('header-scrolled');
                }

                elements.css({'height': newH + 'px', 'lineHeight': newH + 'px'});
                logo.css({'maxHeight': newH + 'px'});
            }

        	if(!header.length) return false;

            if(isMobile)
            {
                return false;
            }

            win.scroll(set_height);
            set_height();
    }