Fuzzy sets theory was introduced in 1965 by Lotfy A. Zadeh (Zadeh, 1965) and basically replaces the Aristotelian YES/NO logic with a multi-valued logic. In other words, the interval between Boolean elements 0 and 1 is filled with real numbers. In this way, fuzzy logic can provide an algorithm with the strength to capture uncertainties with a flexibility that resembles human reasoning and facilitates a heuristic approach in modeling phenomena otherwise complicated natural systems. In many applications fuzzy logic has been adopted as a standard method (see examples from Passino and Yurkovich, 1998) but it is still an emerging field in solar energy estimation. A literature survey shows that there are only few models concerning fuzzy modeling of solar radiation (Sen 1998; Santamouris et al. 1999; Gomez and Casanovas, 2003).
Since fuzzy logic is quit different from Boolean logic a short introduction is in place, followed by an application to solar irradiation computation based on air temperature. More about fuzzy logic can be read in Zimmermann (1996) or Passino and Yurkovich (1998).