climate Article The Solar Radiation Climate of Greece Harry D. Kambezidis 1,2   Citation: Kambezidis, H.D. The Solar Radiation Climate of Greece. Climate 2021, 9, 183. https:// doi.org/10.3390/cli9120183 Academic Editors: Salvatore Magazù and Steven McNulty Received: 11 November 2021 Accepted: 11 December 2021 Published: 15 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Emeritus Researcher, Atmospheric Research Team, Institute of Environmental Research and Sustainable Development, National Observatory of Athens, GR-11810 Athens, Greece; harry@noa.gr 2 Research Associate, Laboratory of Soft Energies and Environmental Protection, Department of Mechanical Engineering, University of West Attica, GR-12241 Athens, Greece Abstract: The solar radiation climate of Greece is investigated by using typical meteorological years (TMYs) at 43 locations in Greece based on a period of 10 years (2007–2016). These TMYs include hourly values of global, H g , and diffuse, H d , horizontal irradiances from which the direct, H b , horizontal irradiance is estimated. Use of the diffuse fraction, k d , and the definition of the direct-beam fraction, k b , is made. Solar maps of annual mean H g ,H d ,k d , and k b are prepared over Greece under clear and all skies, which show interesting but explainable patterns. Additionally, the intra-annual and seasonal variabilities of these parameters are presented and regression equations are provided. It is found that H b has a negative linear relationship with k d ; the same applies to H g with respect to k d or with respect to the latitude of the site. It is shown that k d (k b ) can reflect the scattering (absorption) effects of the atmosphere on solar radiation, and, therefore, this parameter can be used as a scattering (absorption) index. An analysis shows that the influence of solar variability (sunspot cycle) on the H g levels over Athens in the period 1953–2018 was less dominant than the anthropogenic (air-pollution) footprint that caused the global dimming effect. Keywords: solar radiation; climate; scattering index; absorption index; Greece 1. Introduction Solar radiation is the primary source for life on Earth as it controls various fields (at- mospheric environment, e.g., [1]; terrestrial ecosystems, e.g., [2]; terrestrial climate, e.g., [3]). Solar radiation is the most abundant renewable energy source; its exploitation started intensively twenty years ago mainly for photovoltaic (PV) installations [4,5]. Fluctuations in the solar radiation intensity are due to changes in the atmospheric constituents [6], variations in the amount and texture of clouds [7], as well as the Sun–Earth geometry variability (Milankovitch theory [8]). Therefore, clouds and atmospheric aerosols are two factors that play a significant role in determining the solar radiation climate at a site on the scale of decades. These two factors vary over space and time, causing an analogous statistical variability in solar radiation, e.g., [9]. The solar radiation climate at a location provides the levels and trends of the global, diffuse, and direct components over a long period of time (usually equal to or longer than 10 years). Some works have been published in the international literature regarding the solar radiation climate at various locations on Earth; indicative studies are for Barcelona, Spain [10], for Alaska, USA [11], for Central Europe [12], for California, USA [13], for Malawi [9], for Sweden [14], for Thailand [15], for Africa [16], and for Athens, Greece [5]. In Greece no such study has been conducted for the whole country, as there in no organised solar radiation network; the only complete solar platform at the moment is the Actinometric Station of the National Observatory of Athens, established in 1952. Therefore, the present work provides an analysis of the solar radiation climate of Greece for the first time. The diffuse fraction, k d , i.e., the ratio of the diffuse horizontal to the global horizontal irradiance, H d /H g , is used. The direct-beam fraction, k b , is analogously defined as the ratio of the direct horizontal irradiance to the global horizontal one, H b /H g , and is also used in the present work. Climate 2021, 9, 183. https://doi.org/10.3390/cli9120183 https://www.mdpi.com/journal/climate