It is mainly wet air, which consists of dry air and water vapor. The content of water vapour in the mixed air has 3 kinds of expression methods. ① Absolute Humidity: it is equal to the mass of the water vapor contained in the unit volume wet air, i.e. the density of water vapor in the wet air. ② Relative humidity 嗘: The ratio of the actual content of water vapour to the maximum possible content at a certain temperature in a unit volume of wet air. ③ Moisture content D0: Also known as the wet volume. It represents the quality of water vapour that coexists with the unit mass of dry air in wet air.