- 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 wetthermometer 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.
Sinceatmospheric pressure , P, depends upon altitude, so does gamma.
At higheraltitude 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.::Tdew = the
dewpoint temperature at which water condenses.::Twet = the temperature of a wet thermometer bulb from which water can evaporate to air.::Tdry = the temperature of a dry thermometer bulb in air.References
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