analysis of ratio of global to extra-terrestrial radiation (clearness index) at some tropical locations in india

An attempt has been made to analyze and evaluate the daily average clearness index (Hg/H0) in terms of daily average cloudiness index (Hd/H0) for three tropical locations in South India (Chennai, Trivandrum, and Visakapatnam). Long term data (15 years, 1993-2007) of measured daily average global and diffuse solar radiation for the locations have been used for this study. Two correlation equations (linear and polynomial) for each location have been developed for clearness index in terms of cloudiness index and found its validity. Performance statistics of the model has been done and applicability of the model is done by comparing the performance statistics with the existing models. It has been found that the proposed model has least error compared with the existing models.