$(function() {
                $(window).scroll(function(){
     var scrollTop = $(window).scrollTop();
     if(scrollTop != 0)
      $('#nav').stop().animate({'opacity':'0.2'},400);
     else 
      $('#nav').stop().animate({'opacity':'1'},400);
    });
    
    $('#nav').hover(
     function (e) {
      var scrollTop = $(window).scrollTop();
      if(scrollTop != 0){
       $('#nav').stop().animate({'opacity':'1'},400);
      }
     },
     function (e) {
      var scrollTop = $(window).scrollTop();
      if(scrollTop != 0){
       $('#nav').stop().animate({'opacity':'0.2'},400);
      }
     }
    );
            });