Resumen
This paper proposes the use of a cubic polynomial to evaluate the saturation vapor pressure (es) in order to transform the psychrometric equation into a cubic equation in Tw (wet bulb temperature). This is done because it is not possible to write Tw as a direct function of dry bulb temperature (T), relative humidity (RH) and pressure (p). Statistical comparisons show that this polynomial is accurasier than other semi-epirical methods used to calculate es. Results of the third degree equation for Tw were compared whit resultant values of an iterative method and they were satisfactory.