It is well known that the vertical wavenumber (m) spectra of temperature and horizontal wind fluctuations have a steep slope (almost proportional to m-3) in the middle atmosphere. These spectra are considered to be due to saturated gravity waves. However, since power spectral analysis assumes sinusoidal waves with constant amplitudes in the height region for which the spectra are calculated, the information on the height position, where the disturbance having each wavenumber is dominant, is lost. In this paper the dominant height position is examined using a recently developed wavelet method. An analysis is made of temperature observation data by radiosondes and wind data by a mesosphere, stratosphere, and troposphere (MST) radar in the lower stratosphere. It is shown that the height where the variance is largest increases with the wavenumber. This variation with wavenumber is explained well by a monochromatic inertia-gravity wave whose vertical wavenumber changes due to the vertical shear of the large-scale background wind during its vertical propagation. Thus we consider theoretically the vertical wavenumber spectrum of a single saturated gravity wave which propagates upward in a large-scale background wind with a vertical shear. The slope of the theoretical spectrum is steep (proportional to m-4 or m-3 depending on m for a linear shear) and similar to the observations. The result suggests that the vertical shear of large-scale wind structure plays an important role in determining the shape of universal m spectra in the middle atmosphere. ¿ American Geophysical Union 1994