How do I convert Relative Humidity into specific humidity
The Engineering Toolbox gives the following equation for converting relative humidity to specific humidity.


  If we know the relative humidity of the moist air and the water vapor density and density of the air, the specific humidity can be expressed as:


x = 0.622 φ ρws / (ρ - ρws) 100%         (1)

where

x = specific humidity of air vapor mixture (kg/kg)

φ = relative humidity (%)

ρws = density of water vapor (kg/m3)

ρ = density of the moist or humid air (kg/m3)