Based on the Clausius-Clapeyron equation and the ideal gas hypothesis for the gas phase, an explicit equation is derived for the saturation temperature of humid air. In order to account for the real gas effects in the vapor phase, the expression is then modified by using the state equation of a pure water two-phase (vapor-liquid) system at equilibrium recommended by Wagner and Pruss. Unlike other equations, it does not require knowledge of the relative humidity. The new equation describes the saturation temperature of humid air as a function of humidity ratio x and system pressure p in the following ranges: 0 < x < 0.2; 0.5 bar < p < 16 bar. As a side product, a simple expression for relative humidity is deduced.