In this paper, using data obtained for locations in the United Kingdom, we introduce the r Eg = E hg /E Dg versus S o plots, and the r Ed = E hd /E Dd versus S o plots for the hours centred at 0.5 h ..., 1.5 h ..., 2.5 h ... from the solar noon, where E hg is the monthly average hourly global illuminance, E hd the monthly average hourly diffuse illuminance, E Dg the monthly average daily global illuminance, E Dd the monthly average daily diffuse illuminance (all of them on a horizontal surface), and S o the monthly average daily number of hours from sunrise to sunset. From the best fits obtained, E hg can be computed if E Dg and S o are known, and E hd if E Dd and S o are known.