An extended data set of ground-based measurements of shortwave radiation, and cloud optical depth (COD) has been used to evaluate the surface cloud radiative forcing (CRF) in the shortwave range under overcast conditions (confirmed with sky images) at Granada, Spain. CRF varies in linear way with the logarithm of the COT showing a high correlation. The slope of the regression line (b) exhibits a clear dependence on solar zenith angle (SZA). The change in CRF per COD-unit is the cloud forcing efficiency (CFE), which is defined as the CRF derivate with respect to COD. In this case, CFE=b/COD. Experimental CFE varies between -160Wm-2 per COD-unit for SZA=14° and COD=1, and -0.3Wm-2 per COD-unit for SZA=80° and COD=50. The largest values of CFE are observed at low SZA and low COD. These empirical results are corroborated by radiative transfer simulations carried out by LibRadtran code. © 2013 Elsevier Ltd.