This document discusses gas vapor mixtures and dry air/atmospheric air in air conditioning analysis. It defines dry air as air without moisture and atmospheric air as a mixture of dry air and water vapor. Specific and relative humidity are introduced to quantify the amount of moisture in air. Specific humidity is the ratio of water vapor mass to dry air mass, while relative humidity compares the actual water vapor mass to the saturated mass at the same temperature and pressure. The dew point temperature is defined as the temperature when air is cooled at constant pressure until condensation starts. Adiabatic saturation temperature is related to wet-bulb temperature. Finally, psychrometric charts are introduced to specify the state of air using two intensive properties.