The use of dew-point temperature in humidity calculations

Abstract

The dew-point temperature has a number of desirable features as a means of expressing humidity. The Antoine Equation, log ew = A - B(T + C)-1, where ew is the partial pressure and T is the temperature of saturated aqueous vapor, represents the Goff-Gratch formulation quite well over the range of temperature from 0 to 140 ?F. The pressure ew, in inches of mercury, is obtained by taking the constants A = 6.70282, B = 3150.515 (?F)-1 and C = 391.0 ?F, calculated from values given by Dreisbach. It is shown that the dew-point DP is related to the relative humidity RH by the relation: [equation not included]. Lines of nearly constant positive slope represent constant relative humidity values on graphs of dew-point against temperature. The value of the slope decreases from unity for RH = 100 percent to about 0.76 for RH = 10 percent, corresponding to the linear equation [equation not included]. Psychrometric charts showing dew-point and dry-bulb temperature as coordinates with lines representing constant relative humidity and constant wet-bulb temperature (obtained from the Ferrel Equation) are extremely useful, since given values for any two of these four variables serve to locate a point, from which the values of the other two variables can be read directly.