## 4 Conclusions  
Two years of hourly longwave downward and upward irradiance measurements for an inland location are used in the current study. Firstly, the data were undergone an extensive quality control process which was based on tests proposed by the BSRN group. These tests refer to physically possible limits, extremely rare, and climatological limits as well as comparison tests between the two longwave components and with air and ground surface temperatures. The upward irradiances showed slightly higher values than the climatological limits as suggested by the BSRN group. The slight excess (about 0.4% of data points) of the climatological limits of the upward irradiance could be attributed probably to the higher temperatures which prevail at this location. Therefore, all the upward longwave irradiance data were kept in the analysis. On the other hand, the downward irradiance showed unreasonable very low values during the period May-November 2017 and they did not pass the comparison criteria with air temperature. Therefore, 4128 hourly values of downward longwave irradiance were excluded from the statistical analysis due to instability of the pyrgeometer. Regarding to the time consistency test which assess the difference between successive measurements the available data set showed that the rate of changes between successive hours of the downward and upward irradiances are <50 W m\({}^{-2}\) and <65 W m\({}^{-2}\), respectively. The second objective of this study refers to the statistical analysis of both hourly and daily values of the longwave radiation including their frequency distribution. As shown, a strong variability is observed during the winter and spring season. The maximum daily value of \(L_{dt}\) is 35.2 MJ m\({}^{-2}\) while the respective minimum value is 17.4 MJ m\({}^{-2}\). The maximum daily value of \(L_{up}\) is 48.3 MJ m\({}^{-2}\) and the minimum value is 26.3 MJ m\({}^{-2}\). The annual mean daily downward longwave irradiation is 27.3 MJ m\({}^{-2}\). On the other hand, the annual mean daily upward longwave irradiation is 37.8 MJ m\({}^{-2}\). The higher air and ground surface temperatures are responsible for the higher values of upward longwave radiation at this site (inland), especially during the summer period at which \(L_{up}\) has the highest mean daily values from all the radiation components. As a result, the net longwave irradiation at Athalassa is always  
Figure 20: Comparison of the estimated Equation (21) and the measured daily net longwave radiation, at Athalassa. negative ranging from \(-5.9\) to \(-12.1\) MJ/m\({}^{2}\), which represents the radiative cooling potential. On the other hand, the net shortwave irradiation (\(R_{\text{ns}}\)) ranges between 6.6 and 22.4 MJ/m\({}^{2}\). Therefore, the net daily irradiation is positive ranging from 1.3 to 10.5 MJ/m\({}^{2}\). The pattern of \(R_{\text{n}}\) follows the same as the \(R_{nl}\), i.e., higher values during the summer season. Regarding the frequency distribution of \(L_{\text{dn}}\) irradiances the data showed that it follows a normal distribution function, while \(L_{up}\) follows an almost normal distribution but with a long positive tail. Since air temperatures are higher due to climate change process, we expect that both longwave components would be increased in the future. A final objective was the estimation and evaluation of the performance of various models in estimating the downward and upward longwave irradiances under clear and all-sky conditions during daytime or all the time of the day. For the case of clear sky conditions, the [PERSON]'s and [PERSON]'s models were selected which showed the best performance among five models. For this purpose, local calibrated coefficients were estimated based on a training data set of the period 2013-2014 [9]. Implementing the data set for the period 2016-2017 showed that [PERSON]'s model showed slightly better results than [PERSON]'s model, since RMSE was lower than that [PERSON]'s model.