Modelling radio refractive index in the atmospheric surface layer

Knowledge of the refractive index profile at radio frequencies in the surface layer of the atmosphere is required to predict the performance of terrestrial radio systems, and although a constant gradient of refractivity with height is often assumed, both measurements and theory suggest that gradients in the lowest 20 m of the atmosphere may often be greater than those above this level. For the special case of evaporation ducts over water in a neutral atmosphere, a logarithmic refractivity profile is normally assumed, but a general model that includes both this case and the linear profile as special cases is proposed, which may also be used to approximately model stable and unstable surface atmospheres. This new model may be particularly suited to predicting sub-refractive fading.