$('.jump-to-top').click(function() {
  $('body').animate({ scrollTop: 0 }, 'slow');
});

function getScrollTop() {
  if(typeof pageYOffset!= 'undefined'){
    //most browsers
    return pageYOffset;
  }
  else {
    var B = document.body; //IE 'quirks'
    var D = document.documentElement; //IE with doctype
    D = (D.clientHeight)? D: B;
    return D.scrollTop;
  }
}

$(window).scroll(function() {
  var scrollPos = getScrollTop();
  var windowHeight = $(window).height();

  if((windowHeight - scrollPos) <= (windowHeight / 2)) {
    $('.jump-to-top').fadeIn(1200);
  } else if(scrollPos === 0) {
    $('.jump-to-top').fadeOut(1200);
  } else if((windowHeight - scrollPos) >= (windowHeight / 2)) {
    $('.jump-to-top').fadeOut(1200);
  }
});