Sky Emissivity at Clear Nights in Yunnan, China

In this work, sky thermal radiation fallen on the horizon and inclined collector surface at clear nights were directly measured by pyr-radiometers, and an empirical correlation to estimate the emissivity of clear night sky was obtained by linearly fitting the measured data. Results showed that the sky emissivity at clear nights in Yunnan Highlands of China was slightly lower than that expected from correlations of Berdahl et al. and Tang et al. under climatic conditions during the period of measurements, implying that the actual sky over Yunnan highlands was “cooler” than that predicted by other correlations. Results also showed that atmosphere radiation fallen on the surface inclined at 30o from the horizon was slightly lower than that expected from other empirical correlations, the equivalent sky temperature over all the sky dome in Yunnan highlands was almost uniform, and the atmosphere radiation from the sky dome could be regarded as to be isotropic.