The common presence of gaps in solar irradiance series led to the development of new methods for quantifying uncertainty in evidence, and new schemes of inference from uncertain data. The uncertainty of monthly Global Horizontal solar Irradiation (GHI) averages is assessed in this study by investigating the impact of different amounts of daily data gaps. This paper quantifies the uncertainty in the calculation of monthly GHI values in the presence of daily gaps as a function of the monthly clearness index, k(tm) (ratio of the monthly GHI to its corresponding extraterrestrial solar irradiation at the same plane). The results indicate an inverse relation between k(tm). and GHI uncertainty due to daily data gaps: for the least clear skies (minimum k(tm)), even just a single day missing introduces an uncertainty four times bigger than the best scenario. This difference increases with the number of missing days.