## 1 Introduction  
The arid climatic conditions of the Arabian Peninsula are usually characterized by hot and long summer seasons and dusty and dry weather. In summer, the ambient temperature often exceeds 45 \({}^{\circ}\)C, the intensive solar radiation flux reaches 1200 Wm\({}^{-2}\), and the relative humidity of the air drops below 10% [1]. Therefore, cultivation in greenhouses is necessary to protect crops from such harsh environments and enhance and manage crop production throughout the year [2]. Most of the greenhouse covers in the Arabian Peninsula are made of low-density polyethylene (LDPE) film owing to its relatively good mechanical and optical properties, combined with its competitive market price [3, 4]. LDPE films have high solar and thermal radiation transmittances; this allows easy thermal energy release from the greenhouse and consequently reduces the greenhouse cooling requirement in summer. Several studies have reported different values for LDPE film transmittances in the solar (0.6-0.8) and thermal (0.5-0.75) radiation spectrum ranges. These transmittances depend on the film thickness and chemical composition and thermophysical properties of the film. A serious problem with using plastic films to cover greenhouses in arid regions is their short lifetime, which is severely affected by harsh climatic conditions. The lifetime of the most widely used plastic films varies from one cultivating season to 2-3 years [5]. In the literature, the lifetime of plastic films has been reported to be, on average, four seasons in North-Central Europe and 2-3 seasons in the Mediterranean regions [3; 5], and it is expected to be even lower under a harsh arid climate. Degradation of plastic-film-covered greenhouses is mainly due to their exposure to atmospheric agents such as solar radiation, high air temperature and relative humidity, wind, rainfall, and sandy storm, as in the desert of the Arabian Peninsula, as well as to chemical products used in greenhouses [6; 7]. The degradation process of plastic films includes different interacting mechanisms such as photodegradation (due to the intensive solar radiation flux, including UV irradiance allocated with an extremely high air temperature), chemical degradation (due to reactions of the film with air pollutants and agrochemicals used in the greenhouse), and mechanical degradation (due to bond rupture under the influence of mechanical stress) [8]. The photodegradation mechanism is primarily responsible for the deterioration of the optical properties of films and their optical capacity (i.e., drastic reduction in film transmissivity). Degradation of the total radiative properties, i.e., integrated over the whole solar spectrum wave band (200-2500 nm), of plastic-film-covered greenhouses affected by climatic conditions (i.e., natural aging) is reported in the literature. For example, degradation of the optical properties of two types of LDPE films used to cover two E-W-oriented pitched-roof greenhouses was investigated; their transmittances to light reduced to 7.3% and 13.2% after the 24-month service [4]. The effect of sandy wind on the total transmittance of polyethylene (PE) film used to cover greenhouses was examined in the laboratory to simulate the sub-Saharan climatic conditions (north of Algeria) [9]. A drastic reduction in the film transmittance was observed because of the surface erosion of the film caused by the sandy wind [9]. The effect of dust accumulation on the total transmittance of a plastic film (0.2 mm thick) used to cover a solar heater system under tropical climatic conditions was tested [10]. The solar heater was oriented south, inclined 15\({}^{\circ}\) on the horizontal plane, and exposed to climatic agents for 30 days. The authors [10] reported that dust accumulation of 3.72 g m\({}^{-2}\) reduced the total transmittance by about 12%. Exposing biodegradable plastic-film-covered low tunnels to the climatic conditions of Southern Italy for 140 days reduced the photosynthetically active radiation (PAR) transmissivity of these covers by about 13% [11].