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^[1].

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

$\gamma =$ 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.

T_dew = the dewpoint temperature at which water condenses.

T_wet = the temperature of a wet thermometer bulb from which water can evaporate to air.

T_dry = the temperature of a dry thermometer bulb in air.

References

^ Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. (1998). Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements. FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 92-5-104219-5. http://www.fao.org/docrep/X0490E/x0490e00.HTM. Retrieved 2007-10-08.