The present work considers the thermodynamic behaviour of moist air as a function of the state variables temperature, density and humidity ratio. Moist air is treated as an ideal mixture which is composed of two real gases, air and steam. The state functions of the mixture are expressed using the corresponding explicit relations of the two species from which the three isentropic exponents describing the real gas isentropic change are calculated. Saturated conditions are determined by solving the Vapour Liquid Equilibrium problem for real water. Numerical results show the effect of humidity as well as the effect of pressure on the thermodynamic behaviour of atmospheric air (such as the three isentropic exponents k, kTp, k, the classical isentropic exponent k and the velocity of sound) for a wide range of temperatures and pressures. Furthermore the isentropic change is approximated by explicit relations having mathematical forms similar to those of the ideal gas but with different constants and exponents. The obtained accuracy is remarkable, being better than 0.32%. In this way the isentropic change, of moist air can be computed by simple explicit relations having as independent variable the Mach number and the humidity ratio. The effects of humidity are examined on certain cases of practical interest such as the critical state (M = 1), the impulse function and the mass flow rate.

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