## 1 Introduction
The low tunnels were oriented in the N-S direction, and their results indicated that the reduction in the PAR transmissivity occurred mainly up to 500 MJ m\({}^{-2}\) of cumulative solar radiation incident on the tunnels [11]. The effect of aging and condensation on the total transmittance of three types of PE films (each 140 um thick) used to cover three venlo-type experimental greenhouses was examined under the Eastern Mediterranean climatic conditions [12]. The reduction in the transmittance due to the accumulation of dust and dirt was 8% after three-month exposure [12]. A study was conducted in Samsun, Turkey [13], to investigate the effect of dirt and vapor condensation on the light transmission of four types of PE used to cover eight prototypes, pitched-roof greenhouses, oriented in the E-W direction and exposed for three months to climatic agents. The average loss in light transmission due to dirt and condensation was estimated to be 9-15% [13]. Several studies have been conducted worldwide to investigate the effect of aging and climatic conditions on the optical properties of plastic films covering greenhouses. However, such studies have not been thoroughly performed for an arid climate. Few studies have been conducted under the arid climatic conditions of the Arabian Peninsula. The effect of dust and dirt on the solar transmittance of plastic films covering solar collectors was investigated [14; 15; 16]. It was observed that the harsh environment and dust accumulation drastically reduced the transmittance of the cover sheets, and consequently, the performance of the collectors decreased [14; 15; 16]. Under the arid climatic conditions of Riyadh, Saudi Arabia, degradation of the optical properties of 200 \(\upmu\)m thick PE-film-covered greenhouse prototypes was investigated over a period of 13 months [17]. The cover transmittance deteriorated rapidly during the first three months of exposure; the relative losses of global and PAR solar radiation transmittance were 9% and 15.3%, respectively. A survey of previous literature revealed that: (i) previous studies were conducted worldwide under climatic conditions different from an arid climate; (ii) neither the shape nor orientation of greenhouses was considered as a parameter linked to the degradation characteristics of film covers; (iii) a greenhouse cover consists of multi-surfaces with different orientations, receiving solar radiation with different intensities based on the surface orientation related to the sun position during the day; however, most previous studies did not specify the orientation of greenhouses. Therefore, degradation of the radiative properties of each surface is expected to be different from others depending on its position on the cover. In other words, the service life of each surface in the greenhouse may depend on the surface orientation of the cover; therefore, we have to answer an important question the growers need (i.e., if the farmer needs to replace the whole greenhouse cover surfaces at the end of their service life, or if some surfaces can serve for a longer time to reduce the maintenance cost). In this article, the words \"optical properties\" will be used for the total radiative properties of plastic film covers, integrated over the solar spectrum range (wavelength: 200-2500 nm). The aim of this study was to investigate the effect of the greenhouse shape and its orientation on the degradation characteristics of the optical properties (transmittance \(T\) and reflectance \(R\)) of each surface of the cover. Four greenhouse prototypes in two shapes (pitched-roof and tunnel), covered with 0.2 mm thick LDPE films, were tested. Greenhouses of two prototypes were oriented in the N-S direction and the other two in the E-W direction, and all were exposed for one year to arid climatic agents. Degradation of the optical properties of each surface of the four prototypes was investigated individually and evaluated.