Absolute humidity - gritcsenko/Node GitHub Wiki

Relative humidity (RH)

The ratio of the actual amount of water vapour in the air to the amount it could hold when saturated expressed as a percentage or the ratio of the actual vapour pressure to the saturation vapour pressure expressed as a percentage.

$RH=100 \cdot \frac{e}{e_w}$

Absolute humidity

The mass of water vapour in a unit volume of air. It is a measure of the actual water vapour content of the air.

$AH=\frac{m_v}{V}$

From relative humidity and saturation vapour pressure we can find actual vapour pressure.

$e= e_w \frac{RH}{100}$

Then we can use the general law of perfect gases

$PV=\frac{m}{M}RT$

where

$R=8.3144598(48)[\frac{J}{molK}]=8314.4598(48)[\frac{J}{kmolK}]$

In our case this is

$eV=mR_vT$

water molar mass is

$M=18.01528[\frac{g}{mol}]=18015.28[\frac{g}{kmol}]$

and we've got specific gas constant for water vapour

$R_v=\frac{R}{M}=0.46152265188218[\frac{J}{gK}]$

Thus we can express mass to volume ratio as

$\frac{e}{R_vT}=\frac{m}{V}=AH$

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Article about absolute humidity