Psychrometric Formula

a relationship that permits a determination of vapor pressure from the readings of dry-bulb and wet-bulb thermometers. Mathematically, it is expressed as e = E – AP(t – t_w), where e is the vapor pressure in the air, E is the saturation vapor pressure at temperature t_w of

Figure 1. Dependence of the psychrometer constant A on the velocity of air flow past the thermometer bulb

the wet-bulb thermometer, t is the air temperature, P is the atmospheric pressure, and A is the psychrometer constant, which depends on the design of the psychrometer and, in a more direct way, on the velocity of air flow past the thermometer bulb (see Figure 1).