JQUERY - Fixed header gets tranparent when scrolling down

Fixed header gets transparent when scrolling down.js

// Jquery make a fixed header become transparent as you scroll down
// and revert to not transparent when you scroll back up
// here is a url with more details: http://stackoverflow.com/questions/2921186/scroll-function-jquery

(function() {

	var $header = $('#navbar');

$(window).scroll(function () {

   if(scrollY <= 0){
       $header.animate({
            opacity: 1
       }, 300);
   }
   if(scrollY > 0 && $header.is(':not(:animated)')){
       $header.animate({
            opacity: .75
       }, 300);
    }
 });

// as for making the header fixed, simply have the following in the CSS:

#navbar {
    
    position: fixed;

}