Psychrometric constant

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The psychrometric constant, γ = , 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[1].

\gamma =\frac{ \left( c_p \right)_{air} * P }{ \lambda_v * MW_{ratio} }
γ = psychrometric constant [kPa °C-1],
P = atmospheric pressure [kPa],
λv = latent heat of water vaporization, 2.45 [MJ kg-1],
cp = specific heat of air at constant pressure, [MJ kg-1 °C-1],
MWratio = ratio molecular weight of water vapor/dry air = 0.622.


Both λv and MWratio are constants.
Since atmospheric pressure, P, depends upon altitude, so does γ.
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_{dew}\right] = e \left[ T_{wet} \right]

Actual vapor pressure, e_a = e_s - \gamma * \left( T_{wet} - T_{dry} \right)

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.

[edit] References

  1. ^ 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. Retrieved on 2007-10-08.