| Summary: | A mathematical method for accurately determining the upper and lower diffuse-fraction (k<sub>d</sub>) limits that divide the sky into clear, intermediate, and overcast is developed. Fourteen sites around the world are selected for demonstrating the methodology. The upper and lower k<sub>d</sub> values for these sites are determined from scatter plots of direct-normal solar radiation vs. k<sub>d</sub> pairs over the typical meteorological year of each site. They vary between 0.73 and 0.80 for the upper and between 0.24 and 0.27 for the lower k<sub>d</sub> limits. Plots of sunshine duration (SSD) vs. k<sub>d</sub> are prepared for 12 of the 14 sites. These plots show a decreasing trend in SSD with increasing values of k<sub>d</sub>, as anticipated. According to local climatology, the number of the SSD values in each sky-condition classification varies from site-to-site.
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