Emissivity of clear skies

Abstract

In a recent article Berdahl and Fromberg discussed the thermal radiance of clear skies and presented measured data from three U.S. cities in the form of a correlation of sky emissivity versus surface dewpoint temperature. In this note the authors present additional data and the corresponding improved formula for the sky emissivity. In the prior article, the clear sky emissivity at night was found. The larger ''new'' data set consists of 57 months of data obtained at 6 sites as compared to the ''old'' data set of 11 months of data at 3 sites. At first sight the extension of the analysis to the new data set would seem to be straightforward. However, in order to use the largest possible set of data and to maintain the highest possible accuracy, an indirect analysis of the data is required.