Seasonal dependence of medium-scale atmospheric gravity waves is examined in SuperDARN HF radar data and through analysis of the gravity wave dispersion relation. We found that the probability of gravity wave observation in the HF radar data is highest in winter months and lowest in summer months. The winter probability of observation in a given 2-hour period with sufficient amounts of ground backscatter peaks at about 0.8, while in the summer the probability remains near 0.4. Examination of the dispersion relation shows that there is a seasonal dependence to the altitude profile of the gravity wave's vertical wavelength that may lead to a seasonally dependent reflection of the waves resulting from the mesospheric temperature gradient. The dispersion relation predicts that for waves of a given horizontal wavelength, the minimum wave period that may be transmitted through the mesosphere is longer in the summer than in the winter; the longer the wavelength, the longer the minimum period. Thus some waves that are transmitted through the mesosphere in the winter may not be transmitted in the summer. This seasonal dependence predicted from the dispersion relation is consistent with the HF radar observations. ¿ American Geophysical Union 1996