Sky type classification in Harbin during winter

Sky type classification is a significant element in daylight simulation. As a “winter city”, the daylight climate of Harbin is distinctive, and there is therefore a particular need to determine the most appropriate sky models to establish sky luminance distribution in the city and thereby improve the accuracy of daylight simulations. To determine the closest sky type to that of Harbin during winter, sky luminance data consisting of 145 sky elements were collected by a sky scanner at 10 min intervals from June 2018 to February 2019. The sky luminance data were compared with the 15 standard sky definitions, varying from overcast sky to clear sky, proposed by the Commission International de l’ Eclairage (CIE). The sky with the lowest root-mean-square-error was selected as the most appropriate sky type. The results show that clear skies prevail in Harbin during winter, with an occurrence of 70.56%. The dominant sky type is type 12, with an occurrence of 48.57%, followed by types 11, 10, and 8 with occurrences of 17.67%, 5.81%, and 5.36% respectively. The sum of the occurrences of these four sky types exceeds 77%, which means that they can be regarded as generally representative of Harbin’s skies during winter.