Parametric modelling of density perturbation measurements obtained with the University of Western Ontario's Purple Crow Lidar on 5 nights are used to infer that the typical vertical wavenumber spectrum in the upper stratosphere is dominated by a few quasi-monochromatic waves. In general only 2 of these waves, with growth or decay rates on the order of 1/(14 km) or less, carry most of the spectral energy. These waves are present about half the time on the nights studied. When analyzed using classical spectral techniques these waves appear to form a continuous spectrum. These results may help explain why general circulation models obtain reasonable climatologies when gravity wave parameterization schemes based on a small number of propagating gravity waves are employed. ¿ 1999 American Geophysical Union