Psychrometric constant

The psychrometric constant, $\gamma =$, ralates 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.

Since atmospheric pressure, P, depends upon altitude, so does $\gamma$.
Both $\lambda _{v}$ and $MW_{ratio}$ are constants.
Although $\left(c_{p}\right)_{H_{2}O}$ is constant, varied air composition results in varied $\left(c_{p}\right)_{air}$.