Finding frequency distributions of CIE Standard General Skies from sky illuminance or irradiance

The CIE Standard General Sky divides the luminance distributions of skies into 15 types, ranging from overcast sky to clear sky. Finding the type of sky from the set of CIE Standard General Skies is a difficult task. This is due to the need for accurate luminance measurements over a long period of time because such data are not available in many regions around the world and to the difficulty in obtaining accurate readings for the zenith luminance. This study presents a method whereby the ratio of the vertical sky illuminance (or irradiance) on two surfaces or the ratio of vertical sky illuminance to the horizontal sky illuminance can be used to classify a sky into one of the CIE Standard General Skies.