This model computes thermodynamic properties of moist air from three independent (thermodynamic or/and numerical) state variables. Preferred numerical states are temperature T, pressure p and the reduced composition vector Xi, which contains the water mass fraction only. As an EOS the ideal gas law is used and associated restrictions apply. The model can also be used in the fog region, when moisture is present in its liquid state. However, it is assumed that the liquid water volume is negligible compared to that of the gas phase. Computation of thermal properties is based on property data of dry air and water (source: VDI-Wärmeatlas), respectively. Besides the standard thermodynamic variables absolute and relative humidity, x_water and phi, respectively, are given by the model. Upper case X denotes absolute humidity with respect to mass of moist air while absolute humidity with respect to mass of dry air only is denoted by a lower case x throughout the model. See package description for further information.