Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3). The maximum absolute humidity of warm air at 30°C/86°F is approximately 30g of water vapor – 30g/m3. It is called also specific humidity or humidity ratio.
There are two measures of humidity, which are useful in different ways. Absolute humidity, ω, describes the mass of water vapour contained by a given mass of air.
Note here that ms/V = 1/vs and ma/V = 1/va, and hence the relationship in terms of specific volume also.
The following derivation shows how this formula can be converted from mass fractions to partial pressures:
This relationship is very useful in the analysis of air conditioning systems.
Water molecules are continually being exchanged between liquid and atmospheric water vapor. If the number passing to the vapor state exceeds the number joining the liquid, the result is evaporation. When water passes from the liquid to the vapor state, it will absorb 590 cal of heat from the evaporative surface for every gram of water evaporated. The vapor pressure of the liquid is directly proportional to the temperature. Evaporation will proceed until the air becomes saturated with moisture. The absolute humidity of a given air mass is the number of grams of water per cubic meter of air.
At any given temperature, air can hold a maximum amount of moisture, called the saturation humidity, which is directly proportional to the temperature of the air.
At any given temperature, air can hold a maximum amount of moisture, called the saturation humidity, which is directly proportional to the temperature of the air. The relative humidity for an air mass is the percent ratio of the absolute humidity to the saturation humidity for the temperature of the air mass. As the relative humidity approaches 100%, evaporation ceases.
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