Development of Sky Conditions Observation Method using Whole Sky Camera

We propose the new observation method of sky conditions instead of the existing visual observation. In this paper, we describe mainly how to take the whole sky imageries, the methodology how to discriminate sun, clouds and blue sky areas on the imageries, and the classification of sky conditions which are taken account of weather, sun appearance, cloud existence and sky brightness. As for the discrimination of sky conditions, we use Sky Index (SI) and Brightness Index (BI) calculated from whole sky imageries. SI shows the extent of the blueness and grayscale and BI indicates the extent of the brightness. Sun, cloud and blue sky areas are divided by SI and BI. Moreover, we introduce four sky parameters which can discriminate sun appear or not, fraction of cloud area (cloud cover), the extent of brightness in whole area and the extent of blueness in blue sky area on the whole sky images. Then, multi-temporal whole sky imageries are classified into each item of various sky conditions. The results of sky conditions observation are compared with the data observed at the local meteorological observatory.