Psychrometric constant

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.