Specific Humidity
The mass ratio of water vapor contained within an air parcel relative to the total mass of the moist air parcel.
Governing formula
q = m_v / (m_v + m_d) ≈ 0.622 e / P
SI unit
g/kg (Grams of Water Vapor per kg of Air)
In depth
Unlike relative humidity, specific humidity is a conservative quantity that does not change when an air parcel expands or compresses adiabatically as long as no condensation or evaporation occurs.
Examples in the real world
Tropical air masses carrying high specific humidity (~20 g/kg) compared to polar desert air (~1 g/kg).