Estimation of Sky Thermal Irradiance in Arid Climate under Clear Sky Conditions

Abstract

The sky thermal irradiance (Rsky) is an important factor in meteorological, climatic, and energy balance studies. There are several empirical models for predicting Rsky; however, these are based on the meteorological parameters measured in non-arid climatic conditions and an empirical model for predicting Rsky in arid climates is unavailable. Therefore, this study presents two empirical models for predicting the Rsky under clear sky conditions, common in arid regions such as the Arabian Peninsula. One is based on the sky equivalent emissivity (esky-model) and the other on the sky equivalent temperature (Tsky-model). Meteorological parameters (i.e., air temperature, relative humidity, solar radiation, and downward Rsky) were measured over 3 years and used to develop and validate the proposed models. The results showed that the proposed esky- and Tsky-models had the highest predictive accuracy with a coefficient of determination of 0.86 compared with previously published models. In addition, the proposed esky- and Tsky-models resulted in low values of root mean square and mean absolute percentage errors (7.2 and 7.8 W·m−2 and 1, 2 %, respectively); they were able to precisely predict Rsky at any time in a clear arid climate compared with the prohibitive and time-consuming measurements.