How this calculator works
Dalton partial pressures separate dry air and water vapour. Each component contributes its own ideal-gas density. At the same total pressure and temperature, adding water vapour lowers density because water molecules are lighter than the average dry-air molecule. Enter actual station pressure, not a sea-level weather report. Moisture calculations use a liquid-water saturation fit from -40 through 60 C. The gas mixture is ideal and condensate is excluded.
Formula
rho = (p-e)/(R_d T) + e/(R_v T); e = RH e_s(T) or e_s(T_dew). R_d = 287.042 J/(kg K); R_v = R_d/0.621945.
Worked example
At 0 C and 101325 Pa, dry-air density is about 1.29231969 kg/m3. Halving the absolute pressure halves the density at unchanged temperature.
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Frequently asked questions
Why does humid air have lower density?
At fixed temperature and total pressure, water molecules replace heavier dry-air molecules.
Which pressure belongs here?
Use absolute pressure at the measurement site. Altimeter settings and sea-level pressure need conversion first.
Does this include fog droplets?
No. Suspended liquid water or ice adds mass that is outside this gas-only model.