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Psychrometric constant

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],

λ 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],

M W r a t i o =

ratio molecular weight of water vapor/dry air = 0.622.

Additional recommended knowledge

Essential Laboratory Skills Guide

Safe Weighing Range Ensures Accurate Results

Weighing the right way

Both $λ v$ and $M W r a t i o$ 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.

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. Retrieved on 2007-10-08.

Categories: Chemical properties | Gas laws | Chemical engineering | Physical chemistry | Gases