In any mixture of gases, each gas exerts a partial pressure independent of the other gases. The partial pressure exerted by water vapor is called vapor pressure. If all the water vapor in a closed container of moist air with an initial total pressure p were removed, the final pressure p’ of the dry air alone would be less than p. The vapor pressure e would be the difference between the pressure of the moist air and that of the dry air, or p – p´.
- If the rate of evaporation becomes equal to rate of condensation the system is said to be in dynamic equlibrium
- The heat of vaporization of water Hv (cal/g) varies with temperature but may be determined accurately up to 40oC by where T is the temperature in degrees centigrade.
Hv = 597.3 – 0.56T
Humidity & Dew point
Ratio of the amount of moisture in a given space to the amount the space could contain if saturated.
- Dew Point
The temperature having a saturation vapor pressure equal to the existing vapor pressure e.
The specific humidity, usually expressed in grams per kilogram, is the mass of water vapor per unit mass of moist air.
Measurement of Humidity
Measurements of humidity in the surface layers of the atmosphere are made with a psychrometer, which consists of two thermometers, one with its bulb covered by a jacket of’ clean muslin saturated with water. The thermometers are ventilated by whirling or by use of a fan. Because of the cooling effect of evaporation, the moistened, or wet-bulb, thermometer reads lower than the dry, the difference in degrees being known as the wet-bulb depression. The air temperature and wet-bulb depression are used to obtain various expressions of humidity by reference to special tables.
Time Variation of Humidity
- Atmospheric moisture is at a maximum in summer and at a minimum in winter. Relative humidity, however, varies in reverse.
- Relative humidity, of course, behaves in a manner opposite to that of temperature, being at a maximum in the early morning and at a minimum in the afternoon.