The cloudiness effect on solar radiation was analyzed in this study. Measurements of UV erythemal (UVER) and total shortwave (SW) radiation were registered in Valladolid, Central Spain. Simulations by the radiative transfer model SMARTS 2.9.5 were used to calculate the cloud modification factor (CMF). The dependences of CMF (for UVER and SW ranges) on cloud cover and cloud type were analyzed showing a clear decreasing trend with cloud cover, which was stronger for medium clouds than for high ones. A similar analysis was carried out for the ratio between CMFUVER and CMFSW. This ratio presents a clear influence on cloud cover, solar zenith angle (SZA) and cloud type. The smallest SZAs and the high clouds produce a constant trend close to the unity. However, medium and low SZAs and medium clouds lead to higher attenuation of SW radiation.