The dependence of the distribution of the monthly average hourly diffuse radiation on the values of the daily sunshine fraction

Abstract

Many authors have presented plots of $\text{r}{}_{\mathrm{<mtext>d</mtext>}}\text{=}\text{I}{}_{\mathrm{<mtext>d</mtext>}}\text{/}\text{H}{}_{\mathrm{<mtext>d</mtext>}}\text{vs}\text{S}{}_0$, where $\text{I}{}_{\mathrm{<mtext>d</mtext>}}$ is the monthly average hourly diffuse radiation and $\text{H}{}_{\mathrm{<mtext>d</mtext>}}$ the monthly average daily global radiation, both on a horizontal surface, $\text{S}{}_{\mathrm{)}}$ being the monthly average daily number of possible sunshine hours. These plots are established using all the measured values of the hourly diffuse radiation, independently of the state of the sky. In the present work the dependence of the $\text{r}{}_{\mathrm{<mtext>d</mtext>}}$ vs $\text{S}{}_0$ plots on the values of σ = S/S₀ is studied for Uccle, S being the daily number of sunshine hours and S₀ the astronomical day length. Values of $\text{I}{}_{\mathrm{<mtext>d</mtext>}}$ are used for days with σ = 0; 0 < σ ⩽ 0.1; …; 0.8 < σ ⩽ 0.9; σ > 0.9. For σ > 0.5 and the hours centred at 0.5, 1.5, 2.5, 3.5, 4.5, 5.5 and 6.5 h from the solar noon a dependence of the $\text{r}{}_{\mathrm{<mtext>d</mtext>}}$ vs $\text{S}{}_0$ plots on σ is obtained.