Climatology of the Linke and Unsworth-Monteith Turbidity Parameters for Greece: Introduction to the Notion of a Typical Atmospheric Turbidity Year

Solar rays are attenuated by the Earth's atmosphere. This attenuation can be expressed by the turbidity parameters; two of them are the Linke turbidity factor (T-L) and the Unsworth-Monteith turbidity coefficient (T-UM). In this sudy, both parameters are estimated for 33 sites across Greece, and the notion of a Typical Atmospheric Turbidity Year (TATY) is also introduced. Use of the modified clearness index (k'(t)) is made, while a suggestion for a modified diffuse fraction (k'(d)) is given. The adoption of the four climatic zones in Greece for energy purposes is made, where the variation of T-L and T-UM is studied during a TATY under all and clear-sky conditions. The analysis shows maximum levels in both parameters in late winter-early spring in morning and evening hours, with minimum values at midday. The intra-annual variation of the parameters shows maximum values around March and August and minimum values in summertime and late winter. Maps of annual mean T-L and T-UM values over Greece show persistent minimum values over Peloponnese and maximum values over South Ionian Sea. Linear expressions of T-UM vs. T-L are derived for all sites under all and clear-sky conditions. Finally, linear expressions for k'(d) vs. k'(t) are given for all sites and sky conditions.