XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE Determination of Outdoor Design Temperatures and Relative Humidity for Different Cites in Libya Awad Bodalal Department of Mechanical Engineering, University of Benghazi (UOB) Benghazi, Libya drawadbodalal@gmail.com Salah Masheiti Department of Mechanical Engineering, University of Benghazi (UOB), Benghazi, Libya Salah Masheiti@gmail .com Ayman Alkezza Department of Renewable Energy College of Energy Technologies (COET),Ejkerra, Libya aymankhaalifa@gmail.com Almahdi Alabedi Department of Renewable Energy College of Energy Technologies (COET), Ejkerra, Libya Ejkerra, Libya Abstract— The importance of choosing external climatic parameters such as temperature and relative humidity, which naturally determine the energy consumption of such variables, is of paramount importance, especially if the measurement difference between indoor and outdoor conditions is extreme. Therefore, this study attempts to choose a more accurate design temperature and relative humidity for several Libyan cities based on climatic data over 30 years (1990–2020). By using the data of fifteen different meteorological observation stations, distributed over several climatic regions (the Mediterranean coast in the north and the desert in the south) in this study, By calculating the monthly average temperature and relative humidity over the mentioned years, hence, a code was constructed by MATLAB capable of calculating the monthly average as well as choosing the design temperatures through data analysis, statistically based on the methodology and provisions set by ASHRAE for several Libyan cities for the first time (15 cities) and thus filling a large knowledge gap in the literature, especially calculations of energy consumed in buildings, as well as it was noted that the meteorological data that were analyzed during only two years in a previous study yielded results with a level of accuracy comparable to that of thirty years. Accordingly, it is therefore concluded that the data obtained over two years represents the minimum for any similar future endeavors. Keywords— Design temperatures; Relative Humidity; Outdoor Conditions; Accumulative Frequency; HVAC System. I. INTRODUCTION Outdoor design conditions, as easily measured weather metrics, provide designers with characteristic climatic features of a particular geographic location. Commonly measured data include solar radiation, wind speeds, air temperature, humidity, and precipitation levels. These metrics collectively form the basis for both the sizing/ scaling of HVAC systems and energy requirement estimation for buildings. Given the natural variability of outdoor design conditions, however, designers have often found it difficult to determine a single-valued design parameter to build their calculations upon. The conventional approach in the design of HVAC systems, for example, involves the estimation of peak design loads at specific hours of a single day with indoor and outdoor temperatures. While outdoor design weather data, used in the estimation of building energy requirements, represents the severe /prevalent climatic conditions in a given location. The collection of outdoor temperatures typically involves several meteorological stations experimentally measuring temperatures in a time- based fashion. Statistical methods are subsequently employed on the long-term weather data collected to extract mean temperatures usually reported monthly [1,2]. Other supporting design data may be included depending on the situation. The types and quality of data required for the design conditions may range from a very simple set of design temperatures to detailed descriptions of the local weather conditions. Therefore, this study had to provide data and calculate the outdoor design conditions for 15 cities in Libya distributed North and South to cover most of the Libyan regions. A. Typically Chosen Outdoor Design Conditions First before any practical undertaking concerning the thermal design estimation of buildings is carried out, the different methods and design data required are first ascertained. The typical design parameters necessary for the design of HVAC systems are listed and briefly described below: • Latitude, Longitude, and elevation information (geographic information) • Outdoor dry-bulb (DBT) and wet-bulb (WBT) temperatures • Wind pattern data (prevailing direction and intensity) • Prevailing humidity or moisture content in the specified geographic region