Psychrometric constant

The psychrometric constant $$ \gamma $$ 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.


 * $$ \gamma =\frac{ \left( c_p \right)_{air} *  P }{ \lambda_v * MW_{ratio} } $$


 * $$  \gamma = $$ psychrometric constant [kPa °C−1],


 * P = atmospheric pressure [kPa],


 * $$ \lambda_v = $$ latent heat of water vaporization, 2.45 [MJ kg−1],


 * $$ c_p = $$ specific heat of air at constant pressure, [MJ kg−1 °C−1],


 * $$ MW_{ratio} = $$ ratio molecular weight of water vapor/dry air = 0.622.

Both $$ \lambda_v $$ and $$ MW_{ratio} $$ are constants. Since atmospheric pressure, P, depends upon altitude, so does $$\gamma$$. At higher altitude water evaporates and boils at lower temperature.

Although $$ \left( c_p \right)_{H_2 O} $$ is constant, varied air composition results in varied $$ \left( c_p \right)_{air} $$.

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
Saturated vapor pressure, $$e_s = e \left[ T_{wet}\right]$$ Actual vapor pressure,  $$e_a = e_s - \gamma * \left( T_{dry} - T_{wet} \right) $$


 * here e[T] is 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.