Air VPD and Leaf-to-Air Pressure Difference Are Not Always the Same; illustrative quantitative diagram
Define the quantity before calculating
Leaf temperature need not equal the air reading. Using a different leaf temperature changes the saturation pressure associated with that leaf, while the air’s actual vapor pressure still comes from the air temperature and relative humidity. Keep the two concepts separate.
Use VPD Calculator (Vapor Pressure Deficit) with the units and the basis stated on its form. Check what each input represents before carrying the output into another record.
Read the model explicitly
Saturation pressure e(T) = 0.6108 × exp[17.27T/(T+237.3)] kPa. Air VPD = e(air) × (1−RH fraction); leaf-to-air difference = e(leaf) − e(air)×RH.
The Gardening and crops calculators group sits within Biology calculators. Keep a measured observation distinguishable from a hypothetical input when moving between its tools.
A worked unit check
| Input as labeled on the tool | Illustrative value |
|---|---|
| Measured air temperature (°C) | 25 |
| Measured or explicitly assumed leaf temperature (°C) | 25 |
| Air relative humidity (%) | 60 |
| Output from the illustrative inputs | Value | Unit |
|---|---|---|
| Air VPD | 1.26711 | kPa |
| Leaf-to-air vapor-pressure difference | 1.26711 | kPa |
Record the original input and its unit with this result. Recalculate when the measurement basis changes; changing a unit label after calculation does not convert the quantity. More decimal places do not remove sampling or measurement uncertainty.
A related quantity
Daily Light Integral Calculator addresses a related part of the task. Check its own input definition before carrying a number across.
Continue with Temperature and Humidity Readings Need a Shared Place and Time for the other part of this measurement question.
Return to GreenCalc for other subjects. Keep raw observations, chosen assumptions and calculated quantities identifiable so a later revision does not overwrite the basis of the original record.
By GreenCalc · Updated 2026-11-06