Specific Humidity or Humidity Ratio:

Specific or absolute humidity or humidity ratio or moisture content is described as the ratio of the mass of water vapour along with the mass of dry air in a given volume of the combination. It is mention by the symbol ω.

            ...6.10

here the subscripts a & v refer to dry air & water vapour respectively. Now

Putting for m_v and ma from these expressions in Eq. ( 6.10) ,we get the following

         ...6.11

The unit of ω is kg of water vapour per kg of dry air. Also, as p mention the actual total atmospheric pressure, then from Dalton's law

                                     p = p_a + p_v       ...6.12

So that

         ...6.13

 Referring that the total atmospheric pressure is constant at a specific locality, we can see that

                                        ω = f (p_v)

viz., the specific humidity is only a function of the partial pressure of water vapour.