Theory for moisture transport in air
The Moisture Transport in Air Interface solves for the following equation, in which the moisture content variation is expressed through the transport of vapor concentration, which itself can be expressed as the product of molar mass of water, relative humidity, and vapor saturation concentration:
(4-100)
(4-101)
(4-102)
with the following material properties, fields, and source:
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Μv (SI unit: kg/mol) is the molar mass of water vapor.
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  (dimensionless) is the relative humidity.
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csat (SI unit: mol/m3) is the vapor saturation concentration.
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D (SI unit: m2/s) is the vapor diffusion coefficient in air.
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u (SI unit: m/s) is the air velocity field.
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G (SI unit: kg/(m3⋅s)) is the moisture source (or sink).
The transport of vapor concentration is done by convection and diffusion in moist air.
The moisture is supposed to be constituted only of vapor. In other terms the liquid concentration is null.