Data of irradiance from the sun and the sky on the building surfaces are absolutely necessary for practical planners and engineers in order to design excellent and comfortable environments of building interiors. The direct solar irradiance is always estimated, but the sky irradiance is almost neglected because of the lack of the knowledge of the sky radiance distribution that is the origin of the sky irradiance.The sky irradiance can be obtained as the product of the relative sky radiance and the zenith radiance.An equation which expresses the relative yearly mean sky radiance distribution by the solar altitude to the zenith radiance has been composed based upon the data gained at the IDMP station in Kyushu University in 1994 and 1995 and reported in the preceding paper (part 2). An equation for the yearly mean zenith radiance formulated as the next step is shown in this paper. The equation has been composed as a function of the solar altitude and the frequency of occurrences. The yearly mean irradiance by the solar altitude can be easily calculated by applying the result of this research work, which is surely useful for detailed design of the advanced thermal environments of building interiors.
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