def _prop(self, T, rho, fav):
        """Thermodynamic properties of humid air

        Parameters
        ----------
        T : float
            Temperature, [K]
        rho : float
            Density, [kg/m³]
        fav : dict
            dictionary with helmholtz energy and derivatives

        Returns
        -------
        prop : dict
            Dictionary with thermodynamic properties of humid air:

                * P: Pressure, [MPa]
                * s: Specific entropy, [kJ/kgK]
                * cp: Specific isobaric heat capacity, [kJ/kgK]
                * h: Specific enthalpy, [kJ/kg]
                * g: Specific gibbs energy, [kJ/kg]
                * alfav: Thermal expansion coefficient, [1/K]
                * betas: Isentropic T-P coefficient, [K/MPa]
                * xkappa: Isothermal compressibility, [1/MPa]
                * ks: Isentropic compressibility, [1/MPa]
                * w: Speed of sound, [m/s]

        References
        ----------
        IAPWS, Guideline on an Equation of State for Humid Air in Contact with
        Seawater and Ice, Consistent with the IAPWS Formulation 2008 for the
        Thermodynamic Properties of Seawater, Table 5,
        http://www.iapws.org/relguide/SeaAir.html
        """
        prop = {}
        prop["P"] = rho**2*fav["fird"]/1000                             # Eq T1
        prop["s"] = -fav["firt"]                                        # Eq T2
        prop["cp"] = -T*fav["firtt"]+T*rho*fav["firdt"]**2/(            # Eq T3
            2*fav["fird"]+rho*fav["firdd"])
        prop["h"] = fav["fir"]-T*fav["firt"]+rho*fav["fird"]            # Eq T4
        prop["g"] = fav["fir"]+rho*fav["fird"]                          # Eq T5
        prop["alfav"] = fav["firdt"]/(2*fav["fird"]+rho*fav["firdd"])   # Eq T6
        prop["betas"] = 1000*fav["firdt"]/rho/(                         # Eq T7
            rho*fav["firdt"]**2-fav["firtt"]*(2*fav["fird"]+rho*fav["firdd"]))
        prop["xkappa"] = 1e3/(rho**2*(2*fav["fird"]+rho*fav["firdd"]))  # Eq T8
        prop["ks"] = 1000*fav["firtt"]/rho**2/(                         # Eq T9
            fav["firtt"]*(2*fav["fird"]+rho*fav["firdd"])-rho*fav["firdt"]**2)
        prop["w"] = (rho**2*1000*(fav["firtt"]*fav["firdd"]-fav["firdt"]**2) /
                     fav["firtt"]+2*rho*fav["fird"]*1000)**0.5         # Eq T10
        return prop