Estimation of hourly values of downward atmospheric radiation under cloudless skies during day- and night-time conditions

Summary Several atmospheric radiation models have been developed which provide methods for evaluating the effective emissivity of the atmosphere. Most of these methods are derived for night-time data utilizing local empirical coefficients. There are several, however, which are thought to be more universal in their predictive ability and some of these were tested on a data set registered in Granada from 1983 to 1985, inclusive. The models evaluated in this study are the Brutsaert model, the Berdahl and Martin model and the method proposed by Idso; in the first two cases the original coefficients were used. All the equations showed a good behaviour for night-time data but consistently overestimated day-time measured radiation. These deviations are explained in terms of day-night differences in the effective emissivity regime due to the differing vertical structure of the atmosphere during day and night. An empirical correction term for these differences improves the estimates of these models. The Idso model with a reduction in its independent term provided the best adjustment. Nevertheless, the Berdahl and Martin model with the original coefficients and the day-night correction term provided good results too, deviations being within the experimental error, indicating the universality of its coefficients.With 2 Figures