# Relative humidity

> Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.

- Canonical: https://genetixbay.com/knowledge/glossary/relative-humidity.html
- Also written: RH, humidity, relative humidity %, %RH, humidity level
- 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"

Relative humidity (RH) is the ratio of the actual water vapour pressure in the air to the saturation vapour pressure at the same temperature, as a percentage. Because warm air can hold far more water than cold air, RH depends on temperature: warm a room without adding water and RH falls; cool it and RH rises, reaching 100% at the [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md).

## In practice

Common ranges by stage:

| Stage | RH |
|---|---|
| Clones | 70–85% |
| [Seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) and early veg | 60–70% |
| Late veg | 50–65% |
| Flower | 40–55% |
| Late flower | 40–50% |

High RH in flower raises the risk of [bud rot](https://genetixbay.com/knowledge/glossary/botrytis.md) and [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md); Corredor-Perilla and colleagues (2025) reviewed the effects of high humidity in cannabis. Low RH with high temperature drives water loss faster than roots can supply.

RH alone is an incomplete target. 60% RH at 20 °C and 60% RH at 30 °C produce very different [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), so experienced growers set RH from a VPD target and the current temperature. Cheap hygrometers can be off by 5% or more; check them against a calibrated reference or a two-salt test.

## Units and formula

RH (%) = actual vapour pressure ÷ saturation vapour pressure × 100.

## Don't confuse with

- **Absolute humidity**: grams of water per cubic metre of air.
- **VPD**: the drying power, in kPa.
- **Jar RH in curing**: a proxy for [water activity](https://genetixbay.com/knowledge/glossary/water-activity.md), covered under harvest and cure.

## Related terms

[VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [Absolute humidity](https://genetixbay.com/knowledge/glossary/absolute-humidity.md), [Dew point](https://genetixbay.com/knowledge/glossary/dew-point.md), [Dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md), [Humidifier](https://genetixbay.com/knowledge/glossary/humidifier.md)

## Read more

- [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.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
2. Corredor-Perilla IC, Kwon H, Park J, 2025. High relative humidity in cannabis cultivation (effects on flowering, biomass and cannabinoids). Frontiers in Plant Science 16:1678142. https://doi.org/10.3389/fpls.2025.1678142