Daily average flux-densities of possible direct solar radiation are determined by the Ukrainstev method. With the data thus obtained, problems concerning the theoretical approach based on the Kastrov expression and the variation of the Kastrov coefficient derived through iteration are investigated. In addition, the empirical relation of the coefficient to vapor pressure is explored and the sensitivity to the theoretical expression of the relation found is examined together with the retrieval errors. The results show that the daily average flux-density regime can be calculated quite accurately for various parts of the country by making use of the surface vapor pressure data. On this basis, the spatial and temporal characteristic features of the distribution of possible direct solar radiation are discussed in detail.