[[Humidity unit conversions]] ## Specific humidity = humidity ratio >[!Definition] >The ratio of: >- The mass of water vapor in humid air to the mass of dry air >- The partial pressure of water vapor in the air to the partial pressure of dry air > > Units: % ### Specific humidity by mass Specific humidity = $\frac{m_w}{m_a}$ where $m_w$ = mass of water vapor (kg, lb) and $m_a$ = mass of dry air (kg, lb) ### Specific humidity by pressure Specific humidity = $0.62198 * \frac{p_w}{p_{a}- p_w}$ where $p_w$ = partial pressure of water vapor in moist air (Pa, psi) and $p_a$ = atmospheric pressure of moist air (Pa, psi)