The ratio of the partial pressure of water vapour (P_v) to the saturation pressure of water vapour (P_s) at the same temperature is

Explanation:

Humidity ratio/Specific humidity (ω ) (kg/kg) of a given moist air sample is defined as the ratio of the mass of water vapour (mw) to the mass of dry air (ma) contained in the sample.

\(w = \frac{{{m_w}}}{{{m_a}}} = 0.622\frac{{{P_v}}}{{P - {P_v }}} \)

Relative humidity (ϕ):

It is defined as the ratio of partial pressure of water vapour (pv)in a mixture to the saturation pressure (ps) of pure water at the same temperature of the mixture.

\(\phi = \frac{{{p_v}}}{{{p_{vs}}}}\)

Degree of saturation (μ) is the ratio of the humidity ratio (w) to the humidity ratio of a saturated mixture (ws) at the same temperature and pressure. It is also called saturation ratio or percentage humidity.

\(\begin{array}{l} {\rm{w}} = \frac{{{\rm{kg\;of\;water\;vapour}}}}{{{\rm{kg\;of\;dry\;air}}}} = 0.622\frac{{{p_v}}}{{{p_a} - {p_v}}}\\ \mu = \frac{w}{{{w_s}}} = \frac{{{P_v}}}{{{P_{vs}}}}\left( {\frac{{P - {P_{vs}}}}{{P - {P_v}}}} \right) \end{array}\)