Exploring the Sky Longwave Radiance Distribution in the French Basque Country

Abstract

The atmospheric radiation has a significant impact on the outside surface temperatures, and so on the building heating and cooling loads and on the outdoor thermal comfort. It is considered isotropic in most of the thermal simulation programs, though sky thermography shows that it is strongly anisotropic in the absence of clouds, with a gradient from the zenith to the horizon. The main objective of this work is to propose a model for the sky temperature distribution under all-weather. Its originality is to propose thermograms of the complete sky vault under typical weather conditions. The observed sky temperature distributions are compared to the results of the models by Bliss (1961) and Martin and Berdhal (1984a). The latter allowed for a correlation coefficient higher than 92% for five of the six analyzed sky conditions. The comparison between the radiant power absorbed by a vertical surface at the Earth level using this model or considering an isothermal sky shows differences of up to 16%, resulting in differences of up to 3.5°C in the mean radiant temperature.