We present results from a 14-month study of all-sky camera observations of the Hydroxyl (OH) nightglow made at the Peach Mountain Observatory, Michigan (42.3 ¿N; 83.7 ¿W). Spatial variations in the observed OH airglow images have been used to assess gravity-wave (GW) occurrence frequency at ~85 km altitude as a function of season. A strong seasonal dependence of mesospheric GW activity is observed, with peak activity in the summer months and much reduced activity during the winter months. Gravity waves (as defined by observed coherent variations in relative OH brightnesses of >~7.5) were found to be present on about 70% of the clear-sky nights during the summer months. During the spring, fall, and winter months, however, the observed GW occurrence frequency was very low (<10%). Most of the GWs were observed to propagate towards the eastward hemisphere. We suggest that the tropospherically-generated GWs are anisotropic (eastward) thus passing through to the mesosphere only in the summer and being filtered out by the intervening neutral winds during other seasons. It is also possible that the GWs are able to reach higher altitudes without breaking because of their smaller amplitudes at lower altitudes during the summer season relative to the winter season. ¿ American Geophysical Union 1996