Monthly specific Rietveld's correlations

Using yearly and half-yearly data Rietveld developed a method currently used to estimate monthly average daily global radiation on horizontal surfaces ($\text{H}$). In the present work Rietveld's method is applied to monthly data obtained for 100 European stations. The method is validated for $\text{S}{}_0\text{/}\text{S}\text{≲ 10}$, where $\text{S}{}_0$ is the monthly average day length, and $\text{S}$ is the monthly average daily sunshine duration. To estimate values of $\text{H}$, 12 monthly specific correlations and one correlation valid for all months are obtained for the mentioned data using Rietveld's method. Statistical comparison of monthly specific and non-specific correlations shows that relating the root mean square error, for 70% of the stations monthly specific correlations give better results than the non-specific correlations.