First Correlations for Solar Radiation on Cloudy Days in Italy

Relationship between sky cloud cover and solar radiation is presented in order to give informations about solar radiation attitude varying with cloudiness, useful to optimize solar devices. Meteorological and solar data were recorded by Environmental Technical Physic Laboratories, at University of Rome Tor Vergata from April 15th 2009 to May 31th 2010. Those data were associated to hourly cloud observation recorded in the same site in order to calculate the ratio of global radiation at total cloud amount (N okta) G(N) to global radiation at cloudless sky G(0). The comparison between Kimura and Stephenson (1967) work, and Kasten and Czeplak (1980) parameterization were evaluated to characterize Tor Vergata’s site. The model of clear sky condition proposed by Duffie and Beckman (1980), Spena (1996), and Duchon and O’Malley (1998) were used to determinate the transmission coefficient for global radiation at cloudless sky. Monthly and seasonal formulas has been developed using power and polynomial functions: correlation coefficients were shown for each trend. The correlation coefficient R2=0,9 showed that the best approximation is given by polynomial function. The results has indicated that the curve shape of G(N)/G(0) ratio has a maximum for N=3, almost half cloud cover condition, for Rome’s latitude. The meaning for this behavior is that global radiation grow with scattered radiation growth. This correlation represent a new element useful to characterize Rome’s site and to compare radiation trend with others of different latitudes site, and climate.