Relative Humidity

on . Posted in Fluid Dynamics

Relative humidity, abbreviated as RH, a dimensionless number, is a measure of the amount of moisture present in the air compared to the maximum amount of moisture the air can hold at a specific temperature.  It is determined by the ratio of the actual vapor pressure of water vapor in the air to the saturation vapor pressure at the same temperature.  The saturation vapor pressure represents the maximum amount of water vapor the air can hold before it becomes saturated and condensation occurs.

When the air is holding the maximum amount of moisture it can hold at a particular temperature, the relative humidity is 100% and the air is said to be saturated.  If the relative humidity is less than 100%, the air is not saturated and has the potential to hold more moisture.  If the relative humidity reaches 100% and the air temperature drops, condensation occurs, leading to the formation of dew, fog, or clouds.

Relative humidity plays a crucial role in human comfort and health, as high humidity levels can make the air feel hot and sticky, while low humidity levels can cause dryness and discomfort.  It also influences various atmospheric and weather phenomena, including cloud formation, precipitation, and the rate of evaporation.  Relative humidity is measured using instruments called hygrometers, which can be mechanical, electronic, or based on various principles such as capacitance or dew point measurement.


Relative Humidity formula

\( RH = ( e \;/\; e_s ) \;100 \)     (Relative Humidity)

\( e = RH \; e_s \;/\; 100 \)

\( e_s = e \; 100 \;/\; RH \)

Symbol English Metric
\( RH \) = relative humidity  \(dimensionless\) 
\( e \) = actual vapor pressure \(lbf \;/\; in^2\) \(Pa\)
\( e_s \) = saturated vapor pressure \(lbf \;/\; in^2\) \(Pa\)


Relative Humidity formula

\( RH = 100 \;  ( 112 - 0.1 \; T + T_d \;/\; 112 + 0.9 \; T )^8   \) 
Symbol English Metric
\( RH \) = relative humidity  \(dimensionless\) 
\( T_d \) = dewpoint \(F\) \(C\)
\( T \) = temperature \(F\) \(C\)


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Tags: Temperature HVAC Efficiency