Provi in questo modo:
Codice:
/*HEADER*/
.header {
position:fixed;
z-index: 9;
top:0;
left:0;
width:100%;
padding:50}
// aggiunto
.dissolvenza{
position:fixed;
width: 100%;
height: 500px;
margin-left:-200px;
margin-top:-400px;
background: -webkit-linear-gradient(bottom, rgba(255,255,255, 0), rgba(255, 255, 255, 1) 45%);
background: -moz-linear-gradient(bottom, rgba(255,255,255, 0), rgba(255, 255, 255, 1) 45%);
background: -ms-linear-gradient(bottom, rgba(255,255,255, 0), rgba(255, 255, 255, 1) 45%);
background: -o-linear-gradient(bottom, rgba(255,255,255, 0), rgba(255, 255, 255, 1) 45%);
background: linear-gradient(bottom, rgba(255,255,255, 0), rgba(255, 255, 255, 1) 45%);
/mettere dopo l'header
<div class="dissolvenza"></div>
Cordiali saluti.