-The fiux-gradient relationship for temperature in the unstably statified atmospheric surface layer is well established, and seems to apply also to other scalars, such as humidity. Throughout the rest of the boundary layer, however, a flux-gradient relationship based on a flux at the surface is not well defined if a significant entrainment flux exists at the boundary-layer top. Wyngaard (see Wyngaard and Brost, 1984; Moeng and Wyngaard, 1984) has developed a model which assumes that a flux-gradient relationship can be defined separately for bottom-up transport due to a flux at the surface, and for top-down transport due to a flux through the boundary-layer top. He has shown, by means of large-eddy numerical simulations, that the model can successfully predict the flux-gradient relationship for scalars throughout most of the boundary layer. We have used aircraft profile measurements of temperature, humidity, and ozone concentration near the top of the boundary layer, and aircraft vertical flux measuremen -The fiux-gradient relationship for temperature in the unstably statified atmospheric surface layer is well established, and seems to apply also to other scalars, such as humidity. Throughout the rest of the boundary layer, however, a flux-gradient relationship based on a flux at the surface is not well defined if a significant entrainment flux exists at the boundary-layer top. Wyngaard (see Wyngaard and Brost, 1984; Moeng and Wyngaard, 1984) has developed a model which assumes that a flux-gradient relationship can be defined separately for bottom-up transport due to a flux at the surface, and for top-down transport due to a flux through the boundary-layer top. He has shown, by means of large-eddy numerical simulations, that the model can successfully predict the flux-gradient relationship for scalars throughout most of the boundary layer. We have used aircraft profile measurements of temperature, humidity, and ozone concentration near the top of the boundary layer, and aircraft vertic al flux measuremen