## 1 Introduction
Every material with a temperature above 0 kelvin also emits thermal radiation. Therefore, the emissivity of an object is equal to the absorptivity at each wavelength and this is because of the low sky temperature. The temperature in external space close to absolute zero kelvin (\(-273.15\)\({}^{\circ}\)C) reliant on the season and location, sky temperature from the earth will be near or below zero degrees celsius. The difference is mainly by water vapour in the sky that has become warm by absorbing infrared radiation emitted by the Earth. The warmed water vapour returns some of the infrared back to the Earth, and this helps keep the Earth warmer than space. Almost all the water vapour in the atmosphere is found in the troposphere, the layer of the atmosphere between the surface and around 10 km where there is no water vapour in stratosphere [18]. Sky temperature can be simplified as an average of the temperature between the ground and the upper troposphere where there is less water vapour. Therefore, the sky temperature is not the temperature of the sky but a number that indicates that the sky is much warmer than space and cooler than near the Earth. One of the methods to measure the sky temperature was by a weather balloon launched from Del Rio, Texas, on 21 September 2008 (altitude about 277 km away from earth surface) [18]. The temperature decreases sharply until an altitude of about 17,500 m, the temperature then begins to rise, this change marks the tropopause, the border between the troposphere (which is where most water vapour resides) and the very dry stratosphere above. The temperature increase in the stratosphere is caused by the ozone layer, which is warmed when it absorbs ultraviolet sunlight [18]. Several studies have been conducted on sky temperature models on the building's thermal performance. These models are associated with local weather conditions and specific sites. As a result, the existing discrepancies between the models have been an essential factor for the improvement of new models for different locations and climates. The current models do not cover all climates and a location despite sky temperature calculation is necessary to assess the radiative heat transfer between the building and the sky dome. Consequently, considering building thermal performance, the calculation of sky temperature is vital and needs to be undertaken precisely. It is worth noting that most of the sky temperature models are approximated due to the lack of accurate measured data [19]. Sky temperature models are related to local weather conditions and specific sites. Therefore, the existing deviations between the models have been a key factor for developing new models for different locations over the years. In general, the current sky temperature models do not cover most of the locations and climates around the world. Different relationships have been suggested over the years but most of these models are approximated due to the absence of detailed measured data. It is worth mentioning that the sky temperature and ambient temperature are different. Temperatures tend to decrease from the ground going skyward, with a resulting sky temperature lower than the air temperature [20]. Moreover, the difference between sky and air temperatures is higher during the summer, especially during clear sky conditions. There are many building thermal simulation software used in building thermal simulation such as TRNSYS (Transient System Simulation Tool) which specializes in complete solar energy system modelling while computational fluid dynamics (CFD) can be used in a wide range of building design from building site layout design to individual room planning and also can be used for active heating, ventilating and air-conditioning (HVAC) system design to passive ventilation study, consistent indoor air quality valuation, and serious fire smoke and toxin control [21]. The evaluation of the effect of sky temperature is relevant to building annual energy requirements.