We present a seasonal-average climatology of the nocturnal variation of temperature and sodium density in the mesopause region (80--110 km) based on 300 nights of observation from 1990--1997 by the sodium resonance lidar in Fort Collins, Colorado (40.6 ¿N, 105 ¿W). Waves with downward phase progression were observed during all seasons, with properties (coherence over many nights, observed half-period, etc.) consistent with the upward-propagating semidiurnal tide. The semidiurnal tide was strong (10 K at 90 km) and stable from November to March with a 30 km vertical wavelength. In April and September, the vertical wavelengths were longer and the amplitude increased rapidly with height above 95 km. From May--August, there is a change in wave structure and low amplitudes at 90 km, possibly due to the diurnal tide. The Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) predicts a dominant semidiurnal tide in good agreement with observations, except for a difference in the phase at equinox. ¿ 1998 American Geophysical Union