Development of a Measuring Method of Cloud Amount on the Basis of Color Temperature of Skylight: Development of an automatic measuring instrument of cloud amount - Part 2 一种自动云量测量仪器基于天空亮度色温的云量测量方法Yasuko KOGA Department of Architecture, Graduate School of Human-Environment Studies, Kyushu University yskktarmbox.nc.kyushu-u.ac.jpAbstract: Clouds have a great influence on daylight and solar radiation incident on the earths surface. Daylight and solar radiation data should be investigated together with the impact of clouds. Meteorological observations of clouds are made on cloud forms, cloud amount, cloud height and the direction of motion. Among them, cloud amount is considered to be the most dominant factor in fluctuations of daylight and solar radiation. Clouds are observed from both the ground and the air. Ground observation of clouds depends mostly on visual means, which causes individual variation in the results. In addition, the visual observation is inappropriate for long-term continuous measurement at frequent intervals. Instrumental observation of clouds is necessary for consistent analyses of the results on different sites. The purpose of this study is to develop an automatic measuring instrument of cloud amount. This study proposes utilization of correlated color temperature as a parameter to distinguish clouds from the blue sky. Cloud amount is measured by scanning the sky and judging each sky element as blue sky or cloud. In order to obtain discrimination criteria between the blue sky and clouds, measurements of correlated color temperature of sky light were carried out under various sky conditions. The correlated color temperature of the blue sky scattered widely over the range above 4000 K and that of clouds ranged roughly from 5000 K to 8000 K with the peak at 6000 K. They were further investigated for three factors: solar altitude, altitude of the sky element and angular distance between the sky element and the sun. For each factor, the discrimination criteria of correlated color temperature were mathematically expressed as the lowest limit for the blue sky and the highest limit for clouds. Cloud amount is calculated from the sum of the solid angles subtended by the sky elements judged as cloud. The sky elements are judged as blue sky or cloud by any one of the discrimination criteria for the three factors. The calculated values of cloud amount agreed well with the values by visual observation. There was a tendency for calculated values to be smaller than the observed values for thin clouds. However, on the whole, this study confirmed the feasibility of measuring cloud amount from correlated color temperature of skylight. The obtained criteria could be objective for the cloud amount measurement at different sites.