Remove class giving transition effect

Question

Guys I have a class called 'Fixed' with a Transition in it, when I add this class it makes a transaction, the problem is that when I remove this class, it is being removed very roughly, do not know how to proceed to give a transaction effect also in the exit of this class, the class Fixed has a linear transition all of 1 second:

var menu = $('.fixa').offset().top + 10;
var $meuMenu = $('fixa');
$(document).on('scroll', function () {
    if (menu <= $(window).scrollTop()) {
        $('.fixa').addClass('fixed');
    } else {
        $('.fixa').removeClass('fixed');
    }
});