The linear theory of geostrophic adjustment establishes that imbalances in the atmosphere will lead to the generation of a spectrum of inertia-gravity waves. In mid-latitudes, inertia-gravity waves will have periods ranging from about 4 minutes for pure gravity waves to twenty hours for pure inertial oscillations. A local Fourier analysis technique is presented for identifying motions at the inertial frequency in hourly boundary-layer wind profiler data at mid-latitudes. This technique is then applied to data from four boundary-layer wind profilers and three sodars operated during the CASES-99 stable boundary layer experiment. Evidence is presented that shows that inertial oscillations with significant amplitudes occur regularly in the atmospheric boundary layer, often coincident with frontal passages. Two case studies involving frontal passages indicate that ageostrophic imbalances associated with these frontal passages ultimately lead to an asymptotic state in which inertial oscillations can be identified. This sequence of events suggests that a geostrophic adjustment process has occurred following the frontal passage.