show header based on scroll

$(document).ready(function() {
  // get basic measures
  $wwidth = $(window).width();
  $wheight = $(window).height();

  // based on scroll, toggle header
   $(window).scroll(function() {
     var y = $(window).scrollTop();
     if (y > $wheight+($wheight/2)) {
       $('header').fadeIn('slow');
     } else if (y <= $wheight+($wheight/2)) {
       $('header').fadeOut('slow');
     }
  });
});

Useful when you have a full height first section.