Specific humidity ($q=w/(1+w)$)

Specific humidity $q$ is the mass fraction of water vapor in moist air. For vapor mass $m_v$ and dry air mass $m_d$,where $w$ is the humidity ratio. Both are dimensionless, but $q$ uses the total moist mass as its denominator.^[ref]

For an ideal gas mixture, substitute $w=ϵe/(p-e)$ to obtainwhere $e$ is the vapor partial pressure, $p$ is total pressure and $M_v,M_d$ are molar masses. At $p=100\mathrm{kPa}$ and $e=2\mathrm{kPa}$, the result is $q\approx 0.01254$, or $12.54g/kg$ of moist air. These are illustrative inputs, not a measured weather observation.

Heating the same closed sample without adding, removing or condensing water leaves $q$ unchanged while relative humidity can change. This helps distinguish a change in atmospheric moisture content from a change in temperature when interpreting evaporation and thermoregulation.