# Dew point

> Dew point is the temperature at which air becomes saturated and water starts condensing; when leaves or buds cool to it, free water forms and mould risk jumps.

- Canonical: https://genetixbay.com/knowledge/glossary/dew-point.html
- Also written: dewpoint, dew-point temperature, Td, condensation point
- 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"

The dew point is the temperature to which air must be cooled, at constant [water content](https://genetixbay.com/knowledge/glossary/moisture-content.md), to reach 100% [relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md). Any surface at or below the dew point collects condensation. Because it depends only on how much water the air holds, it is a more stable measure of moisture than RH.

## In practice

Air at 26 °C (79 °F) and 60% RH has a dew point of about 17.6 °C (64 °F). If lights go off and the room or the inside of a dense [cola](https://genetixbay.com/knowledge/glossary/cola.md) drops to that temperature, water condenses on tissue. Leaves radiating heat to a cool ceiling or wall can sit a degree or two below air temperature, so trouble can start even when the air itself is above the dew point.

Rule of thumb for flower: keep the coldest surface in the canopy at least 3–5 °C (5–9 °F) above the dew point, especially in the first hour after lights-off. Run [dehumidification](https://genetixbay.com/knowledge/glossary/dehumidifier.md) and airflow through the dark period and ramp temperature down slowly. Free water on buds for a few hours is enough for [botrytis](https://genetixbay.com/knowledge/glossary/botrytis.md) spores to [germinate](https://genetixbay.com/knowledge/glossary/germination.md).

## Units and formula

Magnus approximation (°C): Td = 243.04 × γ ÷ (17.625 − γ), where γ = ln(RH ÷ 100) + 17.625 × T ÷ (243.04 + T).

## Don't confuse with

- **Relative humidity**: changes with temperature; dew point does not.
- **Frost point**: the equivalent below 0 °C, where ice forms.

## Related terms

[Relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md), [Absolute humidity](https://genetixbay.com/knowledge/glossary/absolute-humidity.md), [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [DIF](https://genetixbay.com/knowledge/glossary/dif.md), [Dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md)

## Read more

- [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- [Botrytis (bud rot / grey mould) in cannabis](https://genetixbay.com/knowledge/diseases/botrytis-bud-rot.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