The present work investigates the effectiveness of an innovative procedure to calculate the global real sky irradiance of a mountain urban region, the city of Trento (Italy). The proposed methodology improves the predictive Bird's real-sky model by introducing in it both atmospheric parameters, specifically defined for the analyzed site, and a local cloud cover factor, based on experimental data, to calculate the global real sky irradiance. The experimental data have been measured at the meteorological station of the University of Trento located in the city center. At first, a selection of the global irradiance measurements, representative of daily clear-sky conditions of each season, is presented and compared with the corresponding values obtained by the improved Bird's clear-sky model. Making use of the improved procedure, the monthly mean daily irradiation is then calculated and compared both with experimental measurements covering the years from 2003 to 2006 and available models as well as data banks. The results, presented in terms of statistical functions, demonstrate that the generalized calculation procedures usually adopted, also available from commercial software tools, reach a satisfactory accuracy if compared with an experimental methodology approach as the one proposed in this work. (c) 2010 Elsevier Ltd. All rights reserved.