Code

$(document).ready(function(e) {

    var dist = $(document).scrollTop();
    var alt = $('header').height();
    var n;
    var vel = 2;


        menu_scroll();
          function menu_scroll(){
                $(window).scroll(function() {
                    var dist_scroll = $(document).scrollTop();
                    if( dist < dist_scroll)
                    {
                        var n =alt - dist_scroll/vel;
                        $('header').css({height:n +"px"});
                    }
                    else
                    {
                        var n =alt + dist_scroll/vel;
                        $('header').css({height:n+"px"});
                    }
                 });

                }

});