When exterior global/direct and diffuse illuminance, as well as zenith luminance, is regularly measured and recorded, a selection and specification of the most light efficient skylight conditions is needed. A routine representation of all 1- or 5-minute data on the P-G-D diagram can show extreme diffuse, i.e. skylight ratios normalized to the simultaneous extraterrestrial horizontal illuminance level, which indicate the bright cloudy sky conditions. A new method of cluster analysis was developed to select sky conditions associated with sunlight under specific turbidity or cloudiness conditions at actual solar altitudes, while a rough classification of the sky luminance pattern is possible using the ratio of zenith luminance to diffuse illuminance. Some characteristic data recorded in Bratislava, Slovakia during 1995 and during the three year period 1994-1996, demonstrate the parametrization and selection method used, and document detailed dynamic changes occurring during the brightest days.