A 5-year (1991--1995) climatology of mesospheric gravity wave activity observed with the Urbana medium frequency (MF) radar is presented. We focus on the monthly mean horizontal wind variances associated with gravity waves with observed periods between 10 min and 8 hours. These results are compared with the seasonal characteristics from other gravity wave observations. The total wave variance, u'2+v'2, displays a dominant semiannual variation (maxima summer/winter, minima spring/fall) below 87 km with little seasonal variation above 87 km. The zonal wave variance exceeds the meridional wave variance, v'2/u'2<1, except around regions of zonal wind reversal. In all seasons the observed variance scale height is greater than that of a nondissipating wave field. A simple zero mean wind model based on linear saturation theory is found to be inadequate in reproducing the seasonal characteristics of the observed wave variances. An alternative model based on critical layer filtering by a height dependent wind field can reproduce the observed semiannual oscillation in the wave variance given a specific source input. ¿ 1998 American Geophysical Union