Pop up at center of the screen

function centerPopup() {
    var windowWidth = document.documentElement.clientWidth;
    var windowHeight = document.documentElement.clientHeight;
    var top = jQuery("#Content3").position().top; 
    //alert(top);  
    var popupHeight = jQuery("#popupContact").height();
    var popupWidth = jQuery("#popupContact").width();
    jQuery("#popupContact").css({
        "position": "absolute",
        "top": top - 100,
        //"top": 500,
        "left": windowWidth / 2 - popupWidth / 2
    });
    $("#backgroundPopup").css({
        "height": windowHeight
    });
}