dojo.addOnLoad( function() {
    dojo.connect(window, 'onscroll', 'cloudScroll');
    cloudScroll();
} );

function cloudScroll() {
    dojo.query('.clouds div').forEach( 
        function(s) {
            var scrollPercent = 1 - (dijit.getViewport().t / (dojo.doc.height - dijit.getViewport().h));
            var cloudLevel = s.className.match(/level(\d+)/)[1];
            var newY = ((dijit.getViewport().h - (200 * cloudLevel)) * scrollPercent) + (100 * cloudLevel);
            var cloudAnim = dojo.animateProperty( {
                node: s,
                duration: 5000,
                properties: { top: newY },
                easing: dojo.fx.easing.quadOut
            } ).play();
        }
    );
}