The effects of atmospheric turbulence on optical system performance depend primarily on the refractive index structure parameter C2n and the inner scale ?0. A simple model for C2n and ?0 is described and experimentally tested. The model, based on the Monin-Obuhkov similarity theory, was developed by Thiermann and is valid in the atmospheric surface layer over land. The inputs to the model are standard meteorological parameters plus solar irradiance and parameters that characterize the local terrain. Model results are compared with measurements of C2n and ?0 made with a laser scintillometer at a height of 1.8 m. The performance of the model was found to be good during unstable daytime conditions.