We have analyzed the variability of solar radiation in the Mediterranean and Black Sea regions by comparing yearly and monthly averages to long-term average values calculated from the HelioClim-1 database. Daily sums of global horizontal irradiation are considered for 18 years in the period 1985-2004. Standard deviation of yearly sums of global horizontal irradiation shows low interannual variability, being mostly in the range of 4% to 6%. While in arid climate of Northern Africa, Middle East, and Southern Europe standard deviation goes below 4%, values up to 10% are identified along coasts and in mountains. In the least sunny year out of 18, the solar resource was generally never more than 9% below the long-term average, and only in a few regions the radiation deficit reached 15%. The most stable weather is found in summer with standard deviation in June below 12%. The least stable season is winter, with variability higher then 20% in December, and regionally going above 35%. The solar resource has distinctive time and geographical patterns that might affect financing of large photovoltaic systems, as well as management of the distributed electricity generation.