A large data set obtained by Rayleigh lidars during 4 years has been analyzed in order to describe the gravity wave climatology in the 30- to 75-km altitude range at mid-latitude. The lidar data were collected in two sites different with respect to orography, both located in the south of France (44¿N). The seasonal variability of the wave activity, the vertical growth of potential energy density per unit mass, and the power spectral density versus vertical wave number are shown. The seasonal variability of the wave activity is found to be mainly annual, the maximum of activity occurring during winter. A semiannual component, with a secondary maximum in summer, is superposed to the annual cycle above 60-km altitude. The power spectral density increases from the stratosphere to the mesosphere in the entire spectral range. A significant positive correlation is found between the wave activity and the wind intensity in the stratosphere. Finally, some simple hypotheses, in terms of wave sources and wave transmission, are advanced in order to get an insight into the causes of the observed seasonal and geographical variability of the wave activity. ¿1991 American Geophysical Union