The attenuation induced by time averaging of ground based measurements of internal gravity waves is made scale dependent by the Doppler shifts resulting from mean atmospheric motions, leading to some distortion of the experimental spectra versus vertical wavenumber. This spectral distortion mainly affects the large wavenumber range where the spectral level may be strongly underestimated, even for a few minutes averaging time in the case of vertical velocity measurements. Numerical evaluations of the spectral attenuation are presented, assuming a standard model of the random internal wave field. ¿ American Geophysical Union 1991