How this calculator works
Mixing ratio expresses water-vapour mass per dry-air mass. Actual vapour pressure comes from dew point; saturation vapour pressure comes from air temperature. The pressure difference p-e is the dry-air partial pressure. The chart holds actual vapour pressure and total pressure fixed while varying temperature from the dew point upward. Values are ratios in kg/kg, with the chart scaled to g/kg. The liquid-water saturation relation is limited to -40 through 60 C.
Formula
w = 0.621945 e/(p-e); w_s = 0.621945 e_s/(p-e_s); RH = e/e_s. All pressures must use the same unit.
Worked example
At 0 C, dew point 0 C and 101325 Pa, e = 611.2 Pa and w is about 0.00377439 kg/kg, or 3.77439 g/kg. Heating at fixed dew point and pressure leaves w unchanged.
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Frequently asked questions
Is mixing ratio the same as specific humidity?
No. Specific humidity uses total moist-air mass, q = w/(1+w). Mixing ratio uses dry-air mass.
Why must pressure exceed saturation pressure?
Otherwise a positive dry-air partial pressure at saturation cannot exist in this mixture model.
Why is actual mixing ratio flat on the chart?
The chart holds dew point and total pressure fixed, so the vapour-to-dry-air mass ratio stays constant.