The psychrometric constant γ {\displaystyle \gamma } 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 = ( c p ) a i r ∗ P λ v ∗ M W r a t i o {\displaystyle \gamma ={\frac {\left(c_{p}\right)_{air}*P}{\lambda _{v}*MW_{ratio}}}}
γ = {\displaystyle \gamma =} psychrometric constant [kPa °C−1], P = atmospheric pressure [kPa],
λ v = {\displaystyle \lambda _{v}=} latent heat of water vaporization, 2.45 [MJ kg−1],
c p = {\displaystyle c_{p}=} specific heat of air at constant pressure, [MJ kg−1 °C−1],
M W r a t i o = {\displaystyle MW_{ratio}=} ratio molecular weight of water vapor/dry air = 0.622. Both λ v {\displaystyle \lambda _{v}} and M W r a t i o {\displaystyle MW_{ratio}} are constants. Since atmospheric pressure, P, depends upon altitude, so does γ {\displaystyle \gamma } . At higher altitude water evaporates and boils at lower temperature. Although ( c p ) H 2 O {\displaystyle \left(c_{p}\right)_{H_{2}O}} is constant, varied air composition results in varied ( c p ) a i r {\displaystyle \left(c_{p}\right)_{air}} . 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.
Vapor Pressure Estimation Saturation vapor pressure, e s ( T d e w ) = e a {\displaystyle e_{s}(T_{dew})=e_{a}}
Actual vapor pressure, e a = e s ( T w e t ) − γ ( T d r y − T w e t ) {\displaystyle e_{a}=e_{s}(T_{wet})-\gamma \left(T_{dry}-T_{wet}\right)}
here es(T) is saturation vapor pressure as a function of temperature, T. Tdew = the dewpoint temperature at which water condenses. Twet = the temperature of a wet thermometer bulb from which water can evaporate to air. Tdry = the temperature of a dry thermometer bulb in air.
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