A simple, but realistic, three-layer model of the unstable atmospheric boundary layer (ABL) comprising surface layer, deep mixed layer and transition layer, within which geostrophic adjustment takes place, is utilized. Relations for the mixed-layer wind components and velocity defects are derived, with the latter depending upon entrainment, baroclinity, advection and local acceleration. These predicted quantities are compared with observations from three field experiments in the unstable ABL, including conditions of moderate baroclinity, strong horizontal advection and rapid growth of the mixed layer. The observations cover a range in scale-height ratio fh/u*, of approximately 0.025–0.5, and in normalized height h/z0 of 103 to 107. Within the limits of experimental errors in wind speed measurements the observed and predicted wind components are in good agreement, implying an internal equilibrium with the turbulent field and no significant influence from, what are observed to be, strong entrainment and advective effects. They are described simply by mixed-layer “drag” laws also appropriate to a steady, horizontally homogeneous, barotropic ABL. Overall, the observed and predicted velocity defects are in reasonable agreement, and show significant influences of entrainment and advection which far exceed the baroclinity effect. The results demonstrate the applicability of the three-layer model to real atmospheric situations, and imply that the main response of the ABL flow to entrainment and advection is a rotation of the average mixed-layer flow toward the externally imposed pressure-gradient field. Examples are given of the dependence of cross-isobar flow angle upon an entrainment parameter and the scale-height ratio.