The sky thermal irradiance ( R sky ) is an important factor in meteorological, climatic, and energy balance studies. There are several empirical models for predicting R sky ; however, these are based on the meteorological parameters measured in non-arid climatic conditions and an empirical model for predicting R sky in arid climates is unavailable. Therefore, this study presents two empirical models for predicting the R sky under clear sky conditions, common in arid regions such as the Arabian Peninsula. One is based on the sky equivalent emissivity (ε sky -model) and the other on the sky equivalent temperature ( T sky -model). Meteorological parameters (i.e., air temperature, relative humidity, solar radiation, and downward R sky ) were measured over 3 years and used to develop and validate the proposed models. The results showed that the proposed ε sky - and T sky -models had the highest predictive accuracy with a coefficient of determination of 0.86 compared with previously published models. In addition, the proposed ε sky - and T sky -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 R sky at any time in a clear arid climate compared with the prohibitive and time-consuming measurements.

中文翻译：

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晴空条件下干旱气候下天空热辐照度的估算

天空热辐照度 (R sky) 是气象、气候和能量平衡研究中的一个重要因素。有几种用于预测 R 天空的经验模型；然而，这些是基于在非干旱气候条件下测量的气象参数，并且没有用于预测干旱气候中 R 天空的经验模型。因此，本研究提出了两种经验模型，用于预测晴空条件下的 R 天空，这在阿拉伯半岛等干旱地区很常见。一种基于天空等效发射率（ε 天空模型），另一种基于天空等效温度（T 天空模型）。气象参数（即气温、相对湿度、太阳辐射和向下的 R 天空）经过 3 年的测量，用于开发和验证所提出的模型。结果表明，与先前发布的模型相比，所提出的 ε 天空和 T 天空模型具有最高的预测准确度，确定系数为 0.86。此外，所提出的 ε 天空和 T 天空模型导致均方根和平均绝对百分比误差的值较低（分别为 7.2 和 7.8 W·m -2 和 1, 2 %）；与令人望而却步且耗时的测量相比，他们能够在晴朗干旱的气候中随时准确预测 R 天空。