Crop coefficient

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.

$\gamma =\frac{{\left(c_p\right)}_{air}*P}{{\lambda }_v*M{W}_{ratio}}$

$\gamma =$ 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],

$M{W}_{ratio}=$ ratio molecular weight of water vapor/dry air = 0.622.

Both ${\lambda }_v$ and $M{W}_{ratio}$ 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, $e_a=e_s-\gamma *(T_{dry}-T_{wet})$

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.

See also

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References

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