The derivation of the humidity equation, Hs = (Mw/Ma)(Ps/(P-Ps)), is based on the principles of ideal gas mixtures. In this equation, P represents the total pressure, and Ps denotes the saturation pressure of water vapor. The relationship is established by recognizing that the partial pressure of water vapor in saturated air is equal to the equilibrium vapor pressure at the mixture's temperature. Consequently, the mass of water vapor per unit mass of dry air is derived from the ratio of the saturation pressure to the difference between total pressure and saturation pressure, multiplied by the ratio of molecular weights.
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