Atmospheric water vapor (AWV) is an important constituent of the atmosphere due to its strong absorption effect on solar radiation and high variability over space and time according to the meteorological conditions. AWV’s extinction affects solar radiation reaching the ground and especially the direct normal irradiance (DNI). However, there are many solar energy applications for which DNI needs to be evaluated frequently and accurately and precise estimations are achieved when the AWV effect is considered. In this study, the AWV effect on DNI is estimated for different aerosol conditions. The decrease of DNI is between 10 and 30 % for the vast majority of cases and could reach 60 % for extremely high values of aerosol optical depth. The monthly averaged values of AWV for 2000–2014 across the globe, derived from MODIS-Terra, are calculated and, as an example of the year-to-year change, the differences from year 2014 are presented. According to results, the difference from the long-term average could lead to significant differences in the estimated DNI.