## 3 Results and Discussion  
During the experiment period (1 October 2019-1 October 2020), and to link the environmental conditions with the degradation characteristic, meteorological parameters (i.e., solar radiation flux, maximum and minimum ambient air temperature, relative humidity, and wind speed) were taken from the weather station at King Saud University campus, Riyadh (i.e., representing the typical arid climatic conditions). The monthly averages of the maximum and minimum air temperatures (\(T_{\rm am,max}\) and \(T_{\rm am,min}\)), as well as the relative humidity (\(RH\)), were estimated and are depicted in Figure 2a. To be more representative, the incident solar radiation flux (\(SR\), W m\({}^{-2}\)) was integrated over each day (\(DSR\), Equation (4)) and submitted for each month as solar energy incident (\(SE\)) in MJ m\({}^{-2}\), as depicted in Figure 2b. In addition, the monthly average of wind speed (\(V_{\rm w}\)) was illustrated. In an arid climate similar to that in the central region of Saudi Arabia (i.e., the site of the experiment), \(T_{\rm am,max}\) during the experiment in summer (Figure 2a) increased to values close to the upper working temperature limit of the LDPE film cover (50-80 \({}^{\circ}\)C). The low relative humidity (\(RH\)) induces dry weather throughout the year that negatively affects the lifetime of the film and its optical properties (Figure 2a). The intensive solar irradiance in arid regions makes the integrated solar energy incident on the cover surfaces reach 18 MJ m\({}^{-2}\) per month in winter and 29 MJ m\({}^{-2}\) per month in summer (Figure 2b). The degradation of the optical properties of plastic film covers is due to the combined effects of climatic factors, i.e., \(T_{\rm am,max}\), \(SE\), \(V_{\rm w,}\) and \(RH\)). On the \(x\)-axis of the upcoming figures (i.e., exposure time (month)), zero time represents the new samples, where 1 is October, 2 is November, 3 is December, etc. Because all the cover surfaces are exposed to the same weather conditions, the optical properties of the sample (transmittance, \(T_{\rm s}\) and reflectance, \(R_{\rm s}\)) were assumed to be uniform for the surface that the sample was taken. In other words, the samples in Figure 1b,c represent the cover surfaces in terms of optical properties. To show the degradation characteristics of the optical properties of each surface of the cover individually, the time course of the transmittance of each sample (\(T_{\rm s}\)) taken from the cover of the tunnel prototype (s\({}_{1}\)-s\({}_{4}\)), oriented in the N-S and E-W directions, is illustrated in Figure 3a,b, respectively. In a similar manner, Figure 4a,b illustrates the time course of \(T_{\rm s}\) for the samples (s\({}_{1}\)-s\({}_{6}\)) taken from the six surfaces of the duo-pitched roof cover prototype, oriented in the N-S (Figure 4a) and E-W (Figure 4b) directions. Figure 3: Degradation of the sample transmittances (\(T_{\rm s}\)) with the exposure time for the tunnel prototype, oriented in the N-S (**a**) and E-W (**b**) directions under arid climatic conditions. Figure 2: Variation of the meteorological parameters during the year of experiment (1 October 2019–1 October 2020). (**a**) Monthly average of the maximum and minimum ambient air temperature and the relative humidity; (**b**) monthly average of the wind speed and monthly accumulated solar energy. The results in Figures 3 and 4 indicate that the degradation of the cover sample transmittances (\(T_{s}\)) progressed rapidly during the first three months and the last three months of exposure; this is mainly due to the high ambient air temperature (see Figure 2a) and the intensive solar irradiance (see Figure 2b). This result is in accordance with the results of another study conducted in the same region [17]. Intensive solar irradiance in arid regions should have a major effect on the degradation of the cover transmittance.