# Absolute humidity

> Absolute humidity is the actual mass of water vapour in a volume of air, in grams per cubic metre, independent of temperature.

- Canonical: https://genetixbay.com/knowledge/glossary/absolute-humidity.html
- Also written: AH, water vapour density, water vapor density, g/m³ humidity, humidity ratio
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/relative-humidity.md), 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](https://genetixbay.com/knowledge/glossary/dehumidifier.md) 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](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) 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.

## Related terms

[Relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md), [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [Dew point](https://genetixbay.com/knowledge/glossary/dew-point.md), [Dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md)

## Read more

- [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.md)
- [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)

## Sources

1. Allen RG, Pereira LS, Raes D, Smith M, 1998. Crop evapotranspiration: guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56), Chapter 3. FAO. https://www.fao.org/4/x0490e/x0490e07.htm