Quick answer: From greenhouse air temperature (T) and relative humidity (RH), you can calculate everything needed for moisture work. Saturation vapour pressure is es = 0.6108 · exp(17.27T / (T + 237.3)) kPa and actual vapour pressure is e = RH × es. From those you get the dew point (when condensation starts), VPD = es − e (the crop's drying demand), and the humidity ratio x = 622 · e / (P − e) g/kg, which needs atmospheric pressure P and is the number to use for moisture balances and ventilation. Example: 25 °C, 60 % RH at sea level gives e ≈ 1.90 kPa, dew point ≈ 16.7 °C, VPD ≈ 1.27 kPa and x ≈ 11.9 g/kg.
Quick answer
- Dew point: when surfaces get wet.
- VPD: how strongly the air dries leaves.
- Humidity ratio (g/kg): what moisture balances and ventilation use; needs pressure.
The equations
| Quantity | Equation | Unit |
|---|---|---|
| Saturation vapour pressure | es = 0.6108 · exp(17.27T / (T + 237.3)) | kPa |
| Vapour pressure | e = RH × es | kPa |
| Dew point | Td = 237.3 · ln(e/0.6108) / (17.27 − ln(e/0.6108)) | °C |
| VPD (air) | es − e | kPa |
| Humidity ratio | x = 622 · e / (P − e) | g/kg dry air |
| Absolute humidity | AH = 2167 · e / (T + 273.15) | g/m³ |
(Magnus-type approximation, as used in FAO-56; for 0–50 °C it agrees closely with the ASHRAE formulations. Use one pressure unit throughout.)
Worked example 1: 25 °C, 60 % RH, 101.325 kPa
| Step | Result |
|---|---|
| es | 3.168 kPa |
| e = 0.60 × 3.168 | 1.901 kPa |
| Dew point | 16.7 °C |
| VPD | 1.27 kPa |
| x = 622 × 1.901 / (101.325 − 1.901) | 11.9 g/kg |
| AH | 13.8 g/m³ |
Any surface below 16.7 °C (glass at night, cold pipes) will get wet.
Worked example 2: 18 °C, 80 % RH, at altitude (99.0 kPa)
| Step | Result |
|---|---|
| es | 2.064 kPa |
| e | 1.651 kPa |
| Dew point | 14.5 °C |
| VPD | 0.41 kPa |
| x at 99.0 kPa | 10.55 g/kg |
| x at sea level (101.325 kPa) | 10.30 g/kg |
Dew point and VPD don't change with pressure; humidity ratio does. At altitude, use local pressure in moisture balances. See greenhouse humidity by climate.
Using the results
| Question | Use |
|---|---|
| Will leaves or glass get wet? | Dew point vs surface temperature |
| Is the crop transpiring? | VPD (leaf VPD uses leaf temperature) |
| Will venting dry the house? | Compare x inside and outside |
| How much water must be removed? | x difference × dry air flow; see moisture load calculation |
Related: moisture load calculation.
See the VPD chart guide.
Check your inputs
- RH must be 0–100 %; sensor readings near 100 % carry larger uncertainty.
- Temperature and RH from the same sensor and time.
- Record the pressure basis (site altitude).
- A calculated air state doesn't size a dehumidifier by itself; it feeds the load balance.
FAQ
Frequently asked questions
How do I calculate dew point from temperature and RH?
Calculate es, multiply by RH to get e, then use the dew-point equation above.
What is humidity ratio in a greenhouse?
Grams of water vapour per kilogram of dry air; it is the right quantity for comparing indoor and outdoor air.
Does altitude affect greenhouse humidity calculations?
It changes humidity ratio for the same RH and temperature; dew point and VPD are unaffected.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- FAO Irrigation and Drainage Paper 56, Chapter 3 (meteorological data equations).
