Absolute Humidity

on . Posted in Thermodynamics

Absolute humidity, abbreviated as AH, is a measure of the amount of water vapour present in a unit volume of air.  It is one of several ways to quantify the humidity or moisture content of the air.  Absolute humidity provides an actual measurement of the water vapor content in the air, regardless of the air's temperature and pressure.

Key Points about absolute humidity

  • Water Vapor  -  Water vapor is the gaseous form of water, and it is an essential component of the Earth's atmosphere.  It is invisible but plays a crucial role in weather and climate.
  • Unit Volume of Air  -  Absolute humidity is typically expressed in relation to a specific volume of air.
  • Measurement Units  -  Absolute humidity can be measured using different units depending on the region and field of study.

Absolute humidity is distinct from relative humidity, which is another common measure of air moisture.  Relative humidity is expressed as a percentage and represents the ratio of the amount of water vapor present in the air to the maximum amount of water vapor the air can hold at a given temperature.  Relative humidity is influenced by both the absolute humidity and the air temperature.

In various applications, such as meteorology, HVAC, and industrial processes, understanding and monitoring absolute humidity is important for assessing comfort, humidity control, and predicting weather conditions.

The relationship between absolute humidity, temperature, and pressure is described by the ideal gas law and the saturation vapor pressure curve, which indicates the maximum amount of water vapor that air can hold at a given temperature.  As air temperature increases, it can hold more water vapor, so the absolute humidity value can change with temperature even if the actual moisture content remains constant.


Absolute Humidity formula

\( AH =   m_v \;/\; V \)     (Absolute Humidity)

\( m_v =  AH \; V  \)

\( V =  m_v \;/\; AH \)

Symbol English Metric
\( AH \) = absolute humidity \(lbm\;/\;ft^3\) \(g\;/\;m^3\)
\( m_v \) = mass of water vapor \(lbm\) \( kg\)
\( V \) = volume of air \(ft^3 \) \( m^3 \)


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Tags: Temperature