Modification of the IR sky temperature under different atmospheric conditions in an arid region in central Saudi Arabia: Experimental and theoretical justification

Abstract

Sky temperatures that were estimated from a single‐channel IR detector over Riyadh, Saudi Arabia, were analyzed from June 2008 to May 2011. The data were divided into three main categories: clear sky, cloudy sky, and dusty conditions. The observation and the research results were as follows. During periods of clear‐sky conditions, it was found that the sky temperatures depend mainly on the atmospheric water content, the screen level temperature, and the suspended aerosol particles in the atmosphere. Under cloudy conditions, the sky temperature ranges between −37°C and 5°C. The mean sky temperatures in this case are higher than those of the clear‐sky conditions by approximately 11°C to 18°C. The radiative properties of cloudy skies depend on the cloud characteristics and the intervening atmosphere between the ground and the cloud base. The sky temperature during dusty conditions ranged between −20°C and 8.5°C. The study showed that dusty conditions increase the atmospheric temperatures by approximately 17°C to 31°C. The sky temperatures during dusty periods are affected by several factors, such as the air mass properties, which bring the dust, and the dust particle characteristics, such as size, shape, and chemical composition, which are initially determined by the sources from which the dust originated. Theoretical simulations using MODTRAN software were used to investigate the atmospheric thermal radiation spectral distributions in the three categories. The results show that the major changes occurred within the atmospheric window (8–14μm).