Image from Meteorology Today by C. Donald Ahrens.
Absolute humidity is the ratio of the mass of water vapor to the volume occupied by a mixture
of water vapor and dry air. It is measured in grams of water vapor per cubic meter of
Conversion to relative humidity (RH) can only be made when the temperature is known. For instance 15 g/m3 at 17.5°C equals 100% RH, while at 25°C it equals 65% RH, and at 30°C it equals 50% RH.