AN607

18

Rev. 1.7

6. How Humidity is Quantified

Humidity represents the amount of water vapor contained in the air and can be quantified in many ways. Several of

the terms describing humidity are defined differently for meteorology applications and thermodynamics or chemical

engineering applications. For this reason, it is important to understand the context of the application. The following

is a brief description of the most common terms for quantifying humidity.

6.1. Absolute Humidity

Absolute humidity in the context of meteorological applications, sometimes referred to as “volumetric humidity”, is

defined as the mass of water vapor dissolved in a total volume of moist air at a given temperature and pressure.

Typical units are g/m3 or kg/m3. The value of absolute humidity defined in this manner changes with temperature

and pressure and is inconvenient to use in many engineering applications.

Absolute humidity for use in thermodynamics or chemical engineering applications is defined as the ratio of water

vapor mass to dry air mass. Typical units are kg/kg. Other names for this ratio include mass mixing ratio, humidity

ratio, mass fraction or mixing ratio. This quantity is simpler to use and more accurate for mass balance or heat

calculations. Due to the conflicting definitions of absolute humidity, caution is required when using this term.

6.2. Specific Humidity

Specific humidity, Yw, can be defined for meteorology applications as the ratio of water vapor mass per mass of

moist air expressed as g/kg or kg/kg. Specific humidity is constant with changes in temperature and pressure for

conditions above the dew point and is a useful quantity in meteorology. The rate of evaporation of water is directly

proportional to specific humidity.

6.3. Relative Humidity

Relative humidity is the ratio of actual water vapor present in air with the amount of water vapor that would be

present in air at saturation, expressed as a percentage. The official symbol for relative humidity is

 although RH,

%RH, rh, or %rh are commonly used. Relative humidity can be expressed as the ratio of the actual vapor pressure,

6.4. Dew Point

Upon heating, the capacity of air to absorb moisture increases. Consequently, the relative humidity of air

decreases as the air is heated. Conversely, as moist air is cooled, its capacity to absorb moisture decreases and

relative humidity increases. The dew point is the temperature, assuming constant pressure, moist air is saturated

(reaches 100% relative humidity) and cannot absorb additional water. As the temperature is decreased past the

dew point, moisture condenses until the air is saturated (reaches 100% relative humidity) at the new lower

temperature.

6.5. Frost Point

Frost point is the same as dew point over solid ice where the condensate is frost instead of liquid water.

RH (%)

p'

ps'





100



=