When scrolling up, detecting strange behavior

I am trying to animate a menu bar (fixed position) when its scroll down the opacity should change to 0.6, and when the scrollTop () returns 0, its opacity should be 1.

It works fine when I scroll down, but when I scroll up, sometimes the code works multiple times, its opacity stays 0.6.

$(document).ready(function(){ 
    $(window).scroll(function(){ 
        if($(window).scrollTop() > 0){
            $('.navbar').animate({opacity:0.6});
        }
        if($(window).scrollTop()  <= 0){
            $('.navbar').css('opacity','1');
        }
    });
});