Mass Continuity Equation

            The Continuity Equation, applied to the atmosphere, is simply a rendition of the principle of Conservation of Mass, stating that matter can neither be created nor destroyed. Yet implying that, again, for the atmosphere, the [constant] mass may be redistributed. However, air parcels expand and contract as they respond to pressure changes that may alter their volume in one of two ways: those that are associated with sounds waves and those which occur in association with hydrostatic pressure changes; granted that hydrostatic volume changes are only taken into account when the equation is expressed in (x, y, p) coordinates.