Relative Humidity (%)
The ratio of actual water vapor present to the maximum possible at the current temperature, expressed as a percentage. It changes when temperature changes, even if the moisture content stays the same.
Absolute Humidity (g/m³)
The actual mass of water vapor in a given volume of air. Unlike relative humidity, it does not change with temperature — only with the actual amount of moisture in the air.
Specific Humidity (g/kg)
The ratio of water vapor mass to the total mass of moist air. Commonly used in meteorology and HVAC engineering. More useful than absolute humidity for air-mass analysis.