# Leaf VPD

> Leaf VPD is the vapour pressure deficit between the inside of a leaf, at leaf temperature, and the surrounding air; it is what actually drives transpiration.

- Canonical: https://genetixbay.com/knowledge/glossary/leaf-vpd.html
- Also written: LVPD, leaf vapor pressure deficit, leaf vapour pressure deficit, leaf-to-air VPD, leaf-air VPD
- 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"

Leaf VPD (leaf-to-air [vapour pressure deficit](https://genetixbay.com/knowledge/glossary/vpd.md)) is the difference between the saturation vapour pressure inside the leaf, assumed saturated at [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md), and the actual vapour pressure of the surrounding air. Because water moves from the wet leaf interior to the air, leaf VPD is the true driving force for [transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md). Air VPD only equals leaf VPD when the leaf is at air temperature.

## In practice

Leaf temperature differs from air temperature by light source and transpiration. Under [LED](https://genetixbay.com/knowledge/glossary/led.md), leaves commonly run about 1–3 °C below air; under [HPS](https://genetixbay.com/knowledge/glossary/hps.md) they can be near or above air temperature (Nelson and Bugbee 2015). Using air VPD alone can mislead you by several tenths of a kPa.

Example: a grower at 26 °C air and 60% [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) reads an air VPD of 1.35 kPa and thinks the room is dry. An [infrared thermometer](https://genetixbay.com/knowledge/glossary/ir-thermometer.md) shows the leaf at 24 °C. Leaf VPD is about 1.0 kPa, comfortably in the flower range. Raising humidity here would have been the wrong call.

Measure leaf temperature on top-canopy leaves facing the light, several leaves, with an IR thermometer or thermal camera.

## Units and formula

Leaf VPD (kPa) = es(T_leaf) − es(T_air) × RH ÷ 100, with es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)).

## Don't confuse with

- **Air VPD**: uses air temperature for both terms.
- **Leaf temperature offset**: the input; leaf VPD is the result.

## Related terms

[VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [Leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md), [IR thermometer](https://genetixbay.com/knowledge/glossary/ir-thermometer.md), [Relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md)

## Read more

- [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.md)

## Sources

1. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/
2. 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