The [[Pressure|pressure]] of [[Air|air]] can be calculated using the [[Partial Pressure|partial pressures]] of its components. Of interest are the partial pressures of dry air, $P_a$, and of the water [[Gas|vapour]] in the air, $P_v$​. $P=P_a+P_v$ Specific Humidity/Absolute Humidity/Humidity Ratio $(\omega)$ Specific humidity describes the [[Mass|mass]] ratio of water vapour to the mass of air. $\omega=\dfrac{m_v}{m_a}$ $\omega=0.62dfrac{P_v}{P_a}$ $\omega=\dfrac{0.622P_v}{P−P_v}$ Relative humidity, $\phi$, measures the amount of moisture in air relative to the maximum amount of moisture that the air can hold. When air holds the maximum amount of moisture possible it is saturated. The saturation pressure, $P_g$, and saturation mass $m_g$​ are measurements of the water vapour in the air. The saturation pressure is defined as the saturation pressure for water at the dry bulb air temperature, $P_g=P_{sat}∣_T$. $\phi=\dfrac{m_v}{m_g}=\dfrac{P_v}{P_g}$ $\phi=\dfrac{\omega P}{P_g(0.622+\omega)}$