Specific Humidity or Humidity Ratio

SH is defined as the mass of water vapour per unit mass of dry air in a mixture of air and water vapour.

If m_a = mass of dry air, m_w = mass of water vapour

(35.9)

Also we know that

and

(35.10)

(35.11)

or,

(35.12)

or

(35.13)

Where P is the atmospheric pressure

Relative humidity,

(35.14)

or

or

(35.15)

If a mixture of air and superheated (or unsaturated) water vapour is cooled at constant pressure, the partial pressure of each constituent remains constant until the water vapour reaches its saturated state. Further cooling causes condensation. The temperature at which water vapour starts condensing is called the dew point temperature of the mixture. it is equal to the saturation temperature at the partial pressure, of the water vapour in mixture. (see figure35.1).