New formulae for the equivalent night sky emissivity

Abstract The knowledge of the equivalent sky emissivity is important in computations of radiation cooling systems, as well as in the evaluation of the thermal losses in solar collectors. A new empirical formula has been developed which permits the determination of the equivalent nocturnal sky emissivity as a function of the site's altitude, the ambient temperature, the relative humidity and the degree of cloudiness, of mist or of haze present at the time. The formula has been found to agree well with the equivalent nocturnal sky temperature values measured at various Venezuelan sites and other places. Comparison is made with formulae from other investigators. A modification of Brunt's. Elsasser's and Angstrom's formulae for clear nocturnal sky is also suggested.