The nature of the separated flow fields downwind of moderately steep hills of varying crosswind aspect ratios has been examined using models placed in a simulated adiabatic atmospheric boundary layer in a meteorological wind tunnel. The hills ranged from an axisymmetric cone to a two-dimensional ridge. Concentration patterns resulting from sources placed at numerous heights and distances downwind of these hills were examined. Effective stack heights and amplification factors (i.e., ratios of maximum ground-level concentrations in the presence of the hills to those in the absence of the hills) were used to characterize the effects of the hills on plume transport and diffusion. Amplification factors were generally found to increase as the aspect ratio increased and as the source height approached the reattachment streamline (such that the plume was advected toward the ground). The largest amplification factor (A = 11) was observed when the source was placed halfway from the hill center to the reattachment point at a height of 1 1/4 hill heights downwind of the two-dimensional ridge.