Humidity
VPD calculator
Vapour pressure deficit (VPD) is how strongly the air pulls water out of leaves. Enter the air temperature, the relative humidity and, if you have measured it, how much cooler or warmer the leaves are than the air.
Leaf VPD
1.04 kPa
In the ideal range for growing plants (0.45–1.15 kPa).
Ideal 0.45–1.15 kPa
- Dew point
- 17.0 °C (62.6 °F)
- Air VPD (leaf at air temperature)
- 1.04 kPa
- Leaf temperature
- 24.0 °C (75.2 °F), same as the air
- Vapour pressure: saturated at the leaf / in the air
- 2.984 / 1.940 kPa
VPD in plain English
VPD is the gap between the water vapour the air holds and the most it could hold at that temperature, measured in kilopascals (kPa). Relative humidity alone misleads, because warm air holds far more water: at 70% humidity, MSU Extension notes, VPD is about 0.55 kPa at 60 °F (15.6 °C) but 1.45 kPa at 90 °F (32.2 °C).
The saturation pressure comes from the Tetens equation as printed by the FAO: e° = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa, with T in °C. The air's actual vapour pressure is the humidity times e° at the air temperature. Leaf VPD is e° at the leaf temperature minus that. The dew point is the temperature at which the air would be saturated; a leaf at or below it gets wet.
The e-GRO extension alert puts the ideal greenhouse range at 0.45–1.15 kPa. Lower means soft growth and more disease; higher brings wilting and crisp leaf edges.
Sources
- Allen RG, Pereira LS, Raes D, Smith M. Crop evapotranspiration, chapter 3: Meteorological data. FAO Irrigation and Drainage Paper 56, 1998: saturation vapour pressure (Eq. 11, the Tetens form), actual vapour pressure from relative humidity, and the dew point (Eq. 14), with Annex 2 Table 2.3 used to test the numbers.
- Thomas P. Vapor Pressure Deficit? Why Should I Track That? e-GRO Alert 7(24), 2018: the 0.45–1.15 kPa range, what low and high VPD do, and the Argus reference table used to test the numbers.
- Wollaeger H, Runkle E. Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity? Michigan State University Extension, 2015: about 0.3 kPa when rooting cuttings, above 0.5 kPa for growing plants, and the 60 °F and 90 °F example.
Questions
Why does leaf temperature matter?
Water leaves from air spaces inside the leaf, which are saturated at the leaf's own temperature. A leaf cooler than the air has a smaller VPD than the air reading suggests; one warmed by a lamp or sun has a larger one.
What VPD suits cuttings and seedlings?
MSU Extension suggests a low VPD of about 0.3 kPa while cuttings root, so young plants dry out less, then more than 0.5 kPa once they are growing on. Choose "Rooting cuttings, young seedlings" to see that mark on the gauge.
How do I lower a high VPD indoors?
Raise the humidity or lower the temperature, and keep the potting mix moist so roots can keep up. For a very low VPD, the e-GRO alert's advice is the reverse: more air movement and less humidity.