Ground-based observations of atmospheric density profiles to 92 km were obtained for four successive seasons between summer 1989 and spring 1990. These results were obtained with a powerful Rayleigh lidar facility located at Wright Patterson Air Force Base (Dayton, Ohio). This instrument combined at 14-W XeF laser transmitter with a 2.54-m receiver mirror to observe returns from altitudes between 40 and 95 km. Analysis of the scale height dependence of the denisty profiles produced temperatures with a measurement error of about 5 K (~2.5%) at 90 km when the lidar data was averaged for 20 min, and smoothed in height over 2.7 km. Examination of these profiles for the total of 18 nights showed that there often existed in the mesophere a layer of enhanced temperatures when compared with the U.S. standard profile. The layer centroid height was about 85 km for summer and 70 to 75 km for winter. Data obtained for the equinoctial periods showed the amplitude of these layers to be weak. The winter temperature profiles showed evidence for long-period waves passing through the region of the thermal anomaly while the equinox profiles revealed more sporadic wave activity with shorter vertical wavelengths. Both the winter and summer temperature data displayed regions where the observed lapse rate approached the adiabatic lapse rate. In the summer the wave activity near the inversion layer was weak. ¿American Geophysical Union 1994