# Relationship between Specific and Relative Humidity of Air

## Calculating specific humidity of moist air - knowing relative humidity, water vapor and air density

Humidity is the quantity of water vapor present in air. It can be expressed as an absolute, specific or a relative value.

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

x = 0.622 φ ρ_{ws}/ (ρ - ρ_{ws}) 100%(1)

where

x= specific humidity (humidity ratio) of air vapor mixture (kg_{h2o}/kg_{dry_air})

φ= relative humidity (%)

ρ_{ws}= density of water vapor (kg/m^{3})

ρ= density of the moist or humid air (kg/m^{3})