Psychrometric constant

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.

::$\gamma =\frac$

:$\gamma =$ psychrometric constant Pa °C⁻¹

: P = atmospheric pressure Pa

:$\lambda_v =$ latent heat of water vaporization, 2.45 J kg⁻¹

:$c_p =$ specific heat of air at constant pressure, J kg⁻¹ °C⁻¹

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

Both $\lambda_v$ and $MW_$ are constants.

Since atmospheric pressure, P, depends upon altitude, so does $\gamma$.

At higher altitude water evaporates and boils at lower temperature.

Although $\left( c_p \right)_$ is constant, varied air composition results in varied $\left( c_p \right)_$.

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_\right/math>

Actual vapor pressure, $e_a = e_s - \gamma * \left( T_ - T_ \right)$

: here e 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

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Chemical properties
Gas laws
Chemical engineering thermodynamics
Physical chemistry
Gases
Underwater diving physics