A quasigeostrophic model for the frequency-dependent response of the zonal-mean flow to planetary-wave forcing at Northern Hemisphere (NH) midlatitudes is applied to 4-D-Var ECMWF anal. data for six extended winter seasons. The theor. response is a non-linear function of the frequency of the forcing, the thermal damping time a-1, and a scaling parameter m which includes the aspect ratio of the meridional to the vertical length scale of the response. Regression of the calcd. response from the analyses onto the theor. response yields height-dependent ests. for both a-1 and m. The thermal damping time estd. from this dynamical model is about 2 days in the troposphere, 7-10 days in the stratosphere, and 2-4 days in the lower mesosphere. For the stratosphere and lower mesosphere, the ests. lie within the range of existing radiative damping time ests., but for the troposphere they are significantly smaller.