The use of horizontal sky illuminance to classify the CIE Standard General Skies

The CIE Standard General Skies defines the luminance distribution of skies into 15 types, from overcast sky to clear sky. Finding the frequency distributions of such skies is not a simple task, for two reasons. The first is the need for accurate luminance measurements over a long period of time. Such data are not available in many regions around the world. The second is the difficulty in obtaining accurate readings for the zenith luminance in low latitude climates. This paper presents a method in which the horizontal sky illuminance can be used to classify sky type.