The monthly statistical distribution of solar radiation is a good summary of radiation data and is a particularly well studied meteorological input for solar system design tools. The aim of the present study was to prepare the monthly reference distribution of the daily values of clearness index K T from measured data of daily global solar radiation on a horizontal surface for eight stations in Iraq over 6 yr of observation. The Fischer test (F-test) is applied to the data of every single station of months characterized by the same monthly average K T values at a level of significance α = 0.05. The results show that, in every station, the monthly distribution of the daily K T values depends only on their monthly average value and not on the order number of the month during the year. The study also shows that the distribution function of daily K T values collected from different locations, and referring to a given month for several years, depends only on their mean value.