Psychrometric constant

The psychrometric constant formula_1 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.
Both formula_7 and formula_8 are constants.<br>
Since atmospheric pressure, P, depends upon altitude, so does formula_9.<br>
At higher altitude water evaporates and boils at lower temperature.
Although formula_10 is constant, varied air composition results in varied formula_11.
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, formula_12<br>
Actual vapor pressure, formula_13