How this calculator works
Absolute humidity is the density of water vapour alone, not the density of moist air. First estimate saturation vapour pressure over liquid water using the Bolton approximation, then apply relative humidity and the ideal-gas law. Temperature inversion needs both vapour density and partial pressure; relative humidity alone is insufficient. The vapour gas constant is the ASHRAE dry-air gas constant divided by its molecular-mass ratio. The liquid-water fit is limited here to -40 through 60 C, including supercooled-water convention below freezing.
Formula
e_s = 611.2 exp[17.67 t/(t+243.5)] Pa, t in C; e = RH e_s; rho_v = e/(R_v T), T in K; R_v = 287.042/0.621945 J/(kg K).
Worked example
At 0 C and 100% relative humidity, vapour pressure is 611.2 Pa and absolute humidity is about 0.00484843 kg/m3, or 4.84843 g/m3. At 50% it is half as large.
Advertisement
Frequently asked questions
Is absolute humidity the same as mixing ratio?
No. Absolute humidity is water mass per air volume; mixing ratio is water mass per dry-air mass.
Which density unit should I use?
The result is in kg/m3 by default. Multiply by 1000 to express it in g/m3.
Can I use zero water-vapour density?
Yes for a dry state. Temperature cannot be recovered from a zero density because division by zero gives no unique state.