Related Resources: physics

### Humidity Ratio of Air

Humidity Ratio of Air

Humidity ratio W is the ratio of the mass of water vapor to the mass of dry air:

Eq. 1
W = Mw / Mda

W equals the mole fraction ratio xw / xda multiplied by the ratio of molecular masses ( 18.01528 / 28.9645 = 0.62198 ), thus

Eq. 2
W = 0.62198 xw / xda

Specific humidity γ is the ratio of the mass of water vapor to the total mass of the moist air sample:

Eq. 3
γ = Mw / ( Mw + Mda )

In terms of humidity ratio:

Eq. 4
γ = W / ( 1 + W )

Absolute humidity dv , or water vapor density, is the ratio of the mass of the water vapor to the total volume of the sample:

Eq. 5
dv = Mw / V

Humidity density ρ of a moist air mixture is the ratio of total mass to total volume:

Eq.6
ρ = ( Mw + Mda ) / V = ( 1 / v ) ( 1 + W )

where

v = moist air specific volume (ft3 / lbda )

Saturation humidity Ws (t, p) is the humidity ratio of moist air saturated with respect to water (or ice) at a specified temperature t and pressure p.

Degree of humidity saturation m is the ratio of the air humidity ratio W to the humidity ratio of saturated moist air Ws at the same temperature and pressure:

Eq. 7
u = ( W / Ws )|t, p

where the air saturation humidity ratio Ws is

Eq. 8
Ws
= 0.622 pws / ( p - pws)

pws = saturation pressure of water (psia) in the absence of air, at the given temperature t.

Source

• Heat Transfer, A Practical Approach Yunus A. Cengel