The psychrometric constant γ relates the partial pressure of water in air to the air temperature. This lets one interpolate actual vapor pressure from paired dry and wet thermometer bulb temperature readings.
γ = psychrometric constant [kPa °C
−1],P =
atmospheric pressure [kPa],
λ v = latent heat of water vaporization, 2.26 [MJ kg
−1],
c p = specific heat of air at constant pressure, [MJ kg
−1 °C
−1],
M W r a t i o = ratio molecular weight of water vapor/dry air = 0.622.
Both λ v and M W r a t i o are constants.
Since atmospheric pressure, P, depends upon altitude, so does γ .
At higher altitude water evaporates and boils at lower temperature.
Although ( c p ) H 2 O is constant, varied air composition results in varied ( c p ) a i r .
Thus on average, at a given location or altitude, the psychrometric constant is approximately constant. Still, it is worth remembering that weather impacts both atmospheric pressure and composition.
Saturated vapor pressure, e s = e [ T w e t ]
Actual vapor pressure, e a = e s − γ ∗ ( T d r y − T w e t )
here e[T] is vapor pressure as a function of temperature, T.
