Humidity Ratio of Air
Humidity ratio is the ratio between actual mass of water vapor present in moist air - to the mass of dry air
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Humidity ratio can be expressed by mass of water or by the vapor partial pressure in the moist air.
Humidity Ratio by Mass
Humidity ratio can be expressed as the ratio between the actual mass of water vapor present in moist air - to the mass of the dry air. Humidity ratio is normally expressed in kilogram or pounds of water vapor per kilogram or pounds of dry air.
Humidity ratio expressed by mass:
x = mw / ma (1)
where
x = humidity ratio (kgwater/kgair, lbwater/lbdry_air)
mw = mass of water vapor (kg, lb)
ma = mass of dry air (kg, lb)
Humidity Ratio by Vapour Partial Pressure
Humidity ratio can also be expressed with the partial pressure of water vapor:
x = 0.62198 pw / (pa - pw) (2)
where
pw = partial pressure of water vapor in moist air (Pa, psi)
pa = atmospheric pressure of moist air (Pa, psi)
The maximum amount of water vapor in the air is achieved when pw = pws the saturation pressure of water vapor at the actual temperature. (2) can be modified to:
xs = 0.62198 pws / (pa - pws) (3)
where
xs = specific humidity at saturation (kgwater/kgair, lbwater/lbdry_air)
Since the water vapor pressure is small regarding to the atmospheric pressure, the relation between the humidity ratio and the saturation pressure is almost linear.
Maximum specific humidity at some common temperatures:
| Temperature (oC) | Saturation
Pressure of Water Vapor (Pa) |
Maximum Humidity Ratio(kg/kg) |
| 0 | 609.9 | 0.003767 |
| 5 | 870 | 0.005387 |
| 10 | 1225 | 0.007612 |
| 15 | 1701 | 0.01062 |
| 20 | 2333 | 0.014659 |
| 25 | 3130 | 0.019826 |
| 30 | 4234 | 0.027125 |
Note that the saturation pressure of water vapor, - and the maximum humidity ratio, increases dramatically with the air temperature. This important for the capacity of drying processes.
Example - Humidity Ratio of Moist Air
The specific humidity for saturated humid air at 20oC with water vapor partial pressure 2,333 Pa at atmospheric pressure of 101.325 kPa (1013 mbar, 760 mmHg) can be calculated as:
x = 0.622 2,333 (Pa) / (101,325 (Pa) - 2,333 (Pa))
= 0.0147 (kg/kg)
= 14.7 g/kg
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