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 [cite book |last=Allen |first=R.G. |coauthors=Pereira, L.S.; Raes, D.; Smith, M. |title=Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements |url=http://www.fao.org/docrep/X0490E/x0490e00.HTM |accessdate=2007-10-08 |series=FAO Irrigation and drainage paper 56 |year=1998 |publisher=Food and Agriculture Organization of the United Nations |location=Rome, Italy |isbn=92-5-104219-5 ] .

::$gamma\; =frac\{\; left(\; c\_p\; ight)\_\{air\}\; *\; P\; \}\{\; lambda\_v\; *\; MW\_\{ratio\}\; \}$

:$gamma\; =$ 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 $left(\; c\_p\; ight)\_\{H\_2\; O\}$ is constant, varied air composition results in varied $left(\; c\_p\; ight)\_\{air\}$.

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, $e\_s\; =\; e\; left\; [\; T\_\{wet\}\; ight]$
Actual vapor pressure, $e\_a\; =\; e\_s\; -\; gamma\; *\; left(\; T\_\{dry\}\; -\; T\_\{wet\}\; ight)$

: 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