Since the temperature is probably the most registered meteorological variable, correlation models based on air temperature data are especially interesting to estimate monthly average values of solar irradiation in countries with lack of direct measurements. Previous models that correlate monthly average irradiation on horizontal surfaces with air temperature can be improved by means of dimensional analysis. The procedure makes the influence of the altitude, the distance to sea, and a reference temperature more explicit. The model proposed in the present paper seems to be adequate to the data obtained from meteorological stations supported by official organizations in Asturias, a region with a diverse orography on the northern Spanish coast. Comparisons between model predictions, experimental data, and estimations computed by other methods have shown acceptable results. The methodology has also been applied to other four Spanish stations in order to check the procedure in places with different latitude and climatology. (C) 2009 Elsevier Ltd. All rights reserved.