Absolute humidity
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.
The mass of water vapour in a unit volume of air. It is a measure of the actual water vapour content of the air.
From relative humidity and saturation vapour pressure we can find actual vapour pressure.
Then we can use the general law of perfect gases
where
In our case this is
water molar mass is
and we've got specific gas constant for water vapour
Thus we can express mass to volume ratio as