Based on ray tracing in a smooth ionosphere described by the IRI-2016 model we have infered the seasonal-diurnal dynamics of radio noise observed by four mid-latitude HF radars. In the calculations, noise is assumed to propagate from the radar dead zone boundary. Noise absorption along the ray path is simulated from the IRI-2016 electron density, and from the molecular nitrogen density and electron temperatures obtained from the NRLMSISE-00 model. Model results are compared with experimental radar data, and good agreement between the two is demonstrated. The model makes it possible to estimate the amount of absorption in D-and E-layers under average undisturbed conditions. This is important for the retrieval of long term variations in the electron density in the lower ionosphere. The model also makes it feasible to interpret vertical absorption in experimental data, thereby significantly expanding the capability of HF radars to monitor the lower ionosphere.