Fourteen nights of high-resolution Na temperature lidar data were collected at the Urbana Atmospheric Observatory (40¿N, 88¿W) over a 6-month period from January through June 1991. The temperature structure in the 80- to 105-km region exhibited strong variability due to the influence of gravity waves, tides, and sporadic Na layers (Nas). Extreme temperatures of 140 K and 255 K at 90 km were observed. A cooling rate of 8 K/month was exhibited from winter to summer at 85--90 km, and in late May and early June a well-defined cold mesopause formed near 85 km with temperatures typically between 150 and 165 K. This mesopause structure produced an associated enhancement in the Brunt-Vaisala frequency near 90 km. The average Brunt-Vaisala period decreased from 5.2 min during the winter months to 4.3 min during spring and early summer. Na density and temperature perturbations are 180¿ out of phase on the top side of the Na layer and in phase on the bottom side as predicted by Gardner and Shelton (1985). The rms relative temperature and horizontal wind perturbations averaged 4.7% and 19.8 m/s, respectively. ¿ American Geophysical Union 1993