Absolute humidity (AH) is the mass of water vapour per unit volume of air, usually in grams per cubic metre (g/m³). Unlike relative humidity, it does not change when the air warms or cools (ignoring the small expansion of air), so it tells you how much water is actually in the room. Engineers often use the related humidity ratio (grams of water per kilogram of dry air), which is what dehumidifier and HVAC sizing charts are based on.
In practice
Air at 26 °C (79 °F) and 60% RH holds about 14.6 g/m³. Cool that same air to 20 °C (68 °F) at lights-off without removing any water and RH jumps to about 84%, because 20 °C air can hold only about 17.3 g/m³. That is the reason night-time humidity spikes in flower rooms, and why dehumidifiers must keep running after lights-off.
To lower humidity you must physically remove water: exhaust to drier air outside, or dehumidify. Heating only lowers RH, not AH.
Units and formula
AH (g/m³) ≈ 2,165 × e ÷ (T + 273.15), where e is actual vapour pressure in kPa and T is air temperature in °C.
Don't confuse with
- Relative humidity: a percentage of saturation, which shifts with temperature.
- Dew point: the temperature at which the current AH would saturate the air.
