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 of wet thermometer bulb temperature readings.

$\gamma =c_{p}*P/\lambda _{v}*ratio</math:<math>\gamma =$ psychrometric constant [kPa °C^-1],

P atmospheric pressure [kPa],

$\lambda _{v}=$ latent heat of vaporization, 2.45 [MJ kg^-1],

$c_{p}=$ specific heat at constant pressure, [MJ kg^-1 °C^-1],

$ratio=$ ratio molecular weight of water vapour/dry air = 0.622.

Since atmospheric pressure depends upon altitude, so does $\gamma$ .