A method of measuring sky temperature

Abstract For radiation heat exchange the sky is assumed to be a blackbody at some equivalent temperature known as the sky temperature. When correlating measured collector performance with meteorological data it is desirable to have complete information on all the meterological variables, one of which in the sky temperature. A new system for measuring sky temperature based on a heat flow meter has been constructed and calibrated. The heat flow meter is mounted in a constant velocity air stream to minimise variable convective heat transfer effects caused by the wind. The exposed surface of the heat flow meter, which contains a thermocouple embedded in the metal substrate for measuring surface temperature, is coated with a white paint having a low solar absorptance and a high thermal emittance. The lower polished metal surface of the heat flow meter faces a radiation shield. This paper presents the analysis on which the design was based together with methods of calibration and results of measurements made with the device. The results are encouraging and indicate that there is a need for further work to determine the absolute performance of similar devices over a wide range of conditions.