There is significant vertical transport of mass both within and across the base of the U.S. west coast temperature inversion. The vertical flux has been determined from a variety of observations made on Mt. Sutro Tower, in the heart of San Francisco: (1) increased concentration of trace elements in aerosols above the inversion base; (2) correlation between the diurnal variations of the concentration of condensation nuclei below and just above the inversion base; (3) a rate of decrease with height in the concentration of sea salt nuclei that does not change significantly above the inversion base; and (4) mixing of fluorescent particles, released upwind of the tower both below and above the inversion base, across and within the inversion. The vertical transport, approximately 15 cm s−1 is ascribed to the action of stable internal gravity waves moving through a layer of strong vertical shear of the horizontal wind. Horizontal mixing near their crests results in a net vertical pumping of mass. The apparent vertical diffusion coefficient is larger than would be expected for the observed Richardson number, although it decreases with increasing Richardson number.