A large power-aperture product Rayleigh-scatter lidar system has been successfully built and over 175 nights of middle atmosphere temperature measurements have been obtained. The high signal-to-noise ratio of these measurements allows the stability of the air in the upper stratosphere and mesosphere to be determined. A detailed methodology has been developed to attempt to differentiate between lapse rate variations due to photon counting errors and actual geophysical variations. On nights when the geophysical variations are large compared to the photon counting errors, regions of convective stability and instability can be determined at a reasonably high confidence level. Both statistics of the layers and an ''image'' of the layers is presented for the night of May 31, 1996. The measured percentage of unstable layers is in agreement with the predictions of Hines (1991), as is the apparently sporadic formation and distribution of the unstable regions. ¿ American Geophysical Union 1996