This paper presents a series of equations for calculating radiance distribution on clear skies. In this paper, radiance of any sky element is divided into two components. The one is the component caused by primary scattering of direct solar radiation in the atmosphere. The other is the additional component which is caused mainly by secondary scattering of direct solar radiation and caused partly by tertiary and subsequent scattering of direct solar radiation as well as scattering of reflected radiation from the ground. Each component is affected strongly by the extraterrestrial solar irradiance, solar altitude, and transparency of the atmosphere. The equations for calculating these two components have been derived so as to be able to give the values of radiance itself by physically taking account of these three factors.