The absolute humidity (vapour density) ρ_v [kg m^-3] measures the mass of vapour per unit volume.
The specific humidity q= ρ_v/ρ [dimensionless], where ρ is the mass density of the moist air, measures the mass of vapour per unit mass of the mixture. It can easily be shown that
q = ε e/p
Here ε = 0.622 is the ratio of the molecular weight of water vapour (18) to the molecular weight of "dry air" (a mixture principally of N₂ and O₂, with M.W. 28.95). And e is the vapour pressure (defined below) and p is the total pressure.
The vapour pressure, ie. the partial pressure of water vapour, e [Pa = N m^-2]. (Note: can you express a Newton, N, in terms of more basic units, kg, m, s?).
A special concept is that of the "saturation" or "equilibrium" vapour pressure, e*. At any temperature T, a beaker containing liquid and vapour will come to an equilibrium vapour pressure such that there is a balance between the rate of vapourisation of liquid and the rate of condensation of vapour, thus we write e*=e*(T), meaning, e* is a function of T. See reference books for Tables.
The ideal gas law relates e to ρ_v
e [Pa] = ρ_v [kg m^-3] R_v [J kg^-1 K^-1] T [K]
where R_v is the specific gas constant for water vapour, R_v= 462 [J kg^-1 K^-1].
The relative humidity, RH = e / e*(T)

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Last modified 13 March 2016 (last prior modification 7 Jan, 2002).