Leaf temperature can change your VPD estimate. A cooler leaf has a lower saturation vapor pressure, so an estimate based only on air temperature can overstate the vapor pressure difference between leaf and air.
The correction is the leaf offset: leaf temperature minus air temperature. A measured offset is useful; a default is an assumption.
What the offset changes
leaf temperature = air temperature + leaf offset
In the illustrative example of 78°F air and a 75°F leaf, the offset is −3°F. That is a temperature difference, not a fixed reduction in kPa. The VPD chart handles the calculation using temperature, humidity, and the offset.
| Input | How to interpret it |
|---|---|
| Measured leaf and air temperatures | Their difference supplies an offset for the conditions and locations measured. |
| −2°F default (roughly −1°C) | A starting assumption for the calculator, not a measured leaf temperature. |
| 0°F versus −2°F without a measurement | A comparison showing how sensitive the result is to the assumed offset. |
Why the gap varies
Transpiration cools leaves as water evaporates through the stomata. Light, airflow, water availability, and plant condition affect that cooling and the energy reaching the leaf. The offset therefore changes with conditions; growth stage alone cannot supply it.
A leaf can also run warmer than the air. That reading may have several explanations, including radiation load, airflow, water status, or measurement error. It warrants a closer look, but it does not identify a problem or its cause by itself.
Use a reading that represents the canopy
An infrared thermometer provides a leaf-surface reading to compare with an air probe at canopy height. Several readings across plants and canopy positions are more useful than a single spot that may not represent the whole canopy.
Check the reading and sensor position before drawing a conclusion. Curling, color, and growth stage cannot establish a numerical offset.
Use the measured offset in the calculator when available, and read the output as an estimate based on those inputs. Without a measurement, the comparison in the table is more informative than treating the default as a fact.
Michigan State University’s VPD versus relative humidity supplies greenhouse background, not a measured offset for this tent. The chart assumes the inputs represent the air and leaf surfaces being compared.