## 2 Measurements and Quality Control Processes
### Longwave Radiation Quality Control
The formulae involved with temperature relationships have the following ranges:  
\[0.4*\sigma*T_{a}^{4}<L_{dn}<\sigma*T_{a}^{4}+25\text{W}\,\text{m}^{-2} \tag{4}\]  
\[\sigma*(T_{a}-15K)^{4}<L_{up}<\sigma*(T_{a}+25K)^{4} \tag{5}\]  
The lower limit of the comparison test of Equation (4) is formulated in such a way that the value of the effective atmospheric emittance, defined by \(\varepsilon_{a}=L_{dn}/\sigma*T_{a}^{4}\) is not possible to be less than 0.4. On the other hand, a longwave component greater than a black-body emission at surface temperature, is also considered impossible. The comparisons between the two longwave components are also shown in Table 2.  
Figure 2: Marginal plot of the hourly downward and upward longwave irradiances at Athalassa. The scatter plot also shows the boxplots of the two variables. Reference lines are the extremely rare and the climatological limits (1 st and 2 nd level) for the upward and downward longwave irradiances as given by Table 2.  
The results of these comparisons are shown in Figure 2. The marginal plot shows also the variability of the two variables through their boxplots. The boxplot of the downward irradiance is almost symmetrical around the mean value indicating that this variable follows an approximately normal frequency distribution function. On the other hand, the boxplot of the upward longwave irradiance shows a large number of outliers towards the upper tail of the boxplot. As indicated earlier, a small number (0.4%) of the data points of the upward longwave irradiance are above the extremely rare limits (700 W m\({}^{-2}\)). However, it has to be noted that the climatological limits are based on observations measured in the Great Plains of United States, where the climatological conditions may be different from those in Athalassa. Furthermore, it has to be noted that air temperatures have increased by more than 0.5 \({}^{\circ}\)C per decade in the recent years since 1980 due to the climate change process in the region of Eastern Mediterranean and Middle East ([PERSON], 1980; [PERSON], 1981; [PERSON], 1981). Therefore, it is expected that both longwave components will have higher values in the future. The effect of air temperature at the screen level on both longwave irradiances is illustrated in Figure 3. For example, for a range of air temperatures between 15 and 20 \({}^{\circ}\)C, the upward longwave irradiance is around 400 W/m\({}^{2}\), while the downward longwave irradiance ranges mainly between 250 and 350 W/m\({}^{2}\). Generally, \(L_{dn}\) ranges between 200 and 450 W/m\({}^{2}\) during the day, while during the night when temperatures are lower, \(L_{dn}\) has slightly narrower upper limits (200 to 400 W/m\({}^{2}\)). On the other hand, \(L_{up}\) ranges mainly between 300 to 700 W/m\({}^{2}\) during the day, while during the night it has also narrower upper limits (300 to 550 W/m\({}^{2}\)). As shown in Figure 4 the longwave data points, estimated by Equations (4) and (5) are within the ranges as described by the said equations. However, the downward irradiance at Athalassa during the period May-November 2017, showed unreasonable very low values and they did not pass the said criteria. Therefore, 4128 hourly values were excluded from the statistical analysis due to instability of the pyrgeometer. The total number of missing records for upward irradiances is only 1421 as shown by Table 3.  
Figure 3: The effect of air temperature at the screen level on both the downward and upward longwave irradiances. \[0.4*\sigma*T_{a}^{4}<L_{dn}<\sigma*T_{a}^{4}+25\text{W}\,\text{m}^{-2} \tag{4}\]0 (**a**) Downward and (**b**) Upward longwave irradiance testing versus air temperature (Global cross-comparisons, Equations (4) and (5)). Green represents the upper limit and blue the lower limit. The extreme rare limits can be also obtained by the range of values defined by the limits of 3 times of the standard deviation interval from the mean value. Values outside these thresholds should be considered as extreme values.