Why is q used for specific humidity? $q$ is the symbol used for specific humidity in many textbooks and papers (at least in meteorology and climatology, I'm not familiar with it in other disciplines). Where does this symbol come from, and what does it stand for?
The specific humidity q is a quotient - mass of water vapor in mass of moist air. The mass is expressed per volume, i.e. the density of water vapor $\rho_v$ and the density of dry air $\rho_d$ are used for the definition of the quotient $q$ known as specific humidity:

$$ q = \dfrac{\rho_v}{\rho_v+\rho_d} $$