The psychrometric constant
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.
Since atmospheric pressure, P, depends upon altitude, so does
At higher altitude water evaporates and boils at lower temperature.
is constant, varied air composition results in varied
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,
Actual vapor pressure,
{\displaystyle e_{a}=e_{s}-\gamma *\left(T_{dry}-T_{wet}\right)}