The clearness index is among the most significant concepts in solar engineering. It gives more information about the atmospheric characteristics of solar stations in addition to the degree of solar energy potential of stations and their surrounding areas. Most often, the clearness index is related to sunshine duration measurements at a single station without spatial feature searches. In this study, harmonic analysis is used to model the monthly clearness index values recorded at nine stations of Turkey with distinct meteorological conditions. Consequently, the coefficients of harmonics, amplitude variances and phase angles of the harmonics are calculated, and then, maps of the total variances are evaluated for spatial interpolations. It is seen that up to the seventh harmonic, more than 90% of the total variance can be presented. It is shown that the western and eastern parts of Turkey have nearly similar characteristics. The contribution of each harmonic to total variance is calculated, and then, regional variance maps are evaluated. (C) 2001 Elsevier Science Ltd. All rights reserved.