Explorations: Humidity Calculations
Text under the table on page 77 of The AMS Weather Book notes that its figures for relative humidity are not the “exact scientific calculation” meteorologists use for the calculation. The “exact” calculation uses actual and saturated vapor pressures. Discussions of the concept of vapor pressure are found in most college-level meteorology textbooks. The figures in the table are within one or two percentage points of those calculated scientifically.
You can do the exact calculations using the Moisture Conversions section of the Weather Calculator on the El Paso Area NWS Forecast Office’s Web site. If you want to see what’s behind these calculations, each of the calculations on this site has a link to the formulas used.
If you don’t have an actual station pressure, you can use 1021 millibars in moisture calculations. This works well for weather stations near sea level. If you have a station’s reported altimeter setting and if you know the station’s elevation above mean sea level, you can use the Station Pressure Calculator, in the Pressure Conversions section, to determine the actual station pressure.
The AMS Weather Book’s online outtake on barometers explains the different pressure readings.