Renewable Energy 28 (2003) 2147–2156 www.elsevier.com/locate/renene Data bank Diffuse solar irradiation at a location in the Balkan Peninsula A.G. Paliatsos a,* , H.D. Kambezidis b , A. Antoniou c a General Department of Mathematics, Technological and Education Institute of Piraeus, 250 Thivon and P. Ralli, GR-12244 Athens, Greece b Atmospheric Research Team, Institute of Environmental Research and Sustainable Development, National Observatory of Athens, PO Box 20048, GR-11810 Athens, Greece c Department of Computer Systems, Technological and Education Institute of Piraeus, 250 Thivon and P. Ralli, GR-12244 Athens, Greece Received 14 February 2003; accepted 27 February 2003 Abstract This study correlates experimental values with modelled values of global (K T ) and diffuse (K D ) clearness index in Athens, Greece. The experimental values come from measurements of daily global and diffuse solar irradiation on horizontal surface, while the modelled ones are from linear regression expressions fitted to the experimental data. It is found that the correlations give excellent results for nearly all experimental periods considered (1990–2000). The study also tries to correlate K T values with black smoke (BS) concentration. The analysis shows that there is no significant trend in K T , which would reflect an increase or decrease in light scattering by BS.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Global horizontal radiation; Diffuse horizontal irradiance; Clearness index; Regression models; Balkan Peninsula 1. Introduction The amount of solar energy reaching the surface of the earth is a primary source of renewable energy. A pre-requisite for the estimation of the incident solar radiation on horizontal or inclined surface is the inclusion of the sky conditions in the compu- * Corresponding author. Fax: +30-210-545-0962. E-mail address: agpal@in.teipir.gr (A.G. Paliatsos). 0960-1481/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0960-1481(03)00077-6