A theoretical propagation model is developed to quantify the probability density function of the power received on tropospheric transhorizon radio links at midlatitudes. Experimental radiosonde data are used to build a new random model of the refractive index structure constant. This model takes into account the turbulence strength for both long-term and altitude variations. A method is developed to evaluate the received power probability density function by numerical integration through the common volume allowing the consideration of antenna radiation patterns and ground effects. After numerous comparisons between our new theoretical propagation model and the semiempirical one, NBS 101, we have set up an upper limit for the validity of the volume scattering theory. The limit is found near 3 GHz for beyond line of sight radio links in the case of a turbulence described by the Kolmogorov's spectrum.