YielixVPD Temperature and Humidity Explained
Start with conditions at the crop
A greenhouse can show a comfortable relative humidity at an aisle sensor while leaves near a cold wall, sunny canopy or roof experience a different drying force. Vapor pressure deficit (VPD) combines temperature and humidity into one pressure difference; it is a measurement-based estimate, not a crop prescription or universal target band.
Take air temperature and relative-humidity readings near the crop canopy at the same time. If possible, also measure a representative leaf; record sensor location, time, radiation and whether the leaf was shaded or sunlit.
Air VPD and leaf VPD answer different questions
Air VPD compares saturation at measured air temperature with vapor actually in the air. Leaf VPD uses the same surrounding-air vapor pressure but saturation at measured leaf temperature, so it is the more direct check when a reliable leaf measurement is available.
A leaf can be warmer than air under radiation or cooler near glazing or during evaporative cooling. Without a leaf measurement, the VPD calculator assumes leaf temperature equals air temperature and labels that assumption.
Why humidity alone is not enough
Relative humidity is a ratio, not an amount of water vapor or a drying force. At the same 60% RH, warmer air has a higher saturation vapor pressure and a larger deficit than cooler air.
| Air temperature | Relative humidity | Approximate air VPD |
|---|---|---|
| 15 °C | 60% | 0.68 kPa |
| 25 °C | 60% | 1.27 kPa |
Formula and units
The calculator uses the FAO-56 saturation-vapor-pressure equation. Convert Fahrenheit to °C before applying it; pressure outputs are kPa.
es(T) = 0.6108 × exp(17.27 × T / (T + 237.3)) [kPa; T in °C]
ea = es(T_air) × RH / 100 [kPa]
VPD_air = es(T_air) − ea; VPD_leaf = es(T_leaf) − ea
T_air and T_leaf are °C, and RH is percent. Air and leaf equations share ea; only their saturation-temperature term changes.
Worked example: a cooler leaf changes the answer
Use the calculator’s Celsius setting with canopy air 25 °C, relative humidity 60%, and measured leaf temperature 23 °C—the same fixture used by the live calculator tests.
- Air saturation:
es(25) ≈ 3.16778 kPa. - Actual vapor pressure:
ea = 3.16778 × 60 / 100 ≈ 1.90067 kPa. - Air VPD:
3.16778 − 1.90067 ≈ 1.26711 kPa. - Leaf VPD:
es(23) − 1.90067 ≈ 0.90877 kPa.
The cooler leaf is about 0.36 kPa lower than air VPD. Change only leaf temperature to 25 °C: leaf VPD should equal air VPD, showing the effect is the leaf-temperature input.
Enter your canopy air and leaf measurements to compare air and leaf VPD
Greenhouse VPD Calculator
Calculate greenhouse air and leaf vapor pressure deficit (VPD) in kPa from air temperature, relative humidity and optional measured leaf temperature using the FAO-56 equation.
Interpret the result before changing equipment
Negative leaf VPD means calculated air vapor pressure exceeds saturation pressure at the entered leaf temperature: a dew-point/condensation diagnostic, not a complete disease or crop-loss model. Confirm sensors and inspect conditions locally.
VPD does not size equipment. The greenhouse heating cost calculator can budget heating assumptions and the greenhouse fan sizing calculator can estimate ventilation; recheck canopy conditions after a change.
Assumptions and limitations
- Air temperature and RH describe the same canopy air at the same time.
- Leaf interior is saturated at measured leaf temperature; emissivity, aim, boundary layers, airflow and surface wetness can affect the reading.
- FAO-56 is a transparent saturation-pressure estimate over water, not a greenhouse energy, condensation or plant-health model.
Common mistakes
- Entering Fahrenheit as Celsius.
- Using RH without temperature or assuming every leaf equals air temperature.
- Clamping a negative leaf VPD result to zero instead of checking for condensation conditions.
- Treating a color band or target as universal without a crop- and stage-specific source.
Sources
The saturation-vapor-pressure equation and RH relationship are from FAO Irrigation and Drainage Paper 56, Chapter 3, Meteorological Data. Crop targets and operational changes still need crop-, stage- and greenhouse-specific verification.