Vapor pressure deficit (VPD) is how hard the air pulls water out of a leaf. Set the room's temperature and humidity, and how much cooler the leaves run than the air, to get leaf VPD and where it sits against the target for a growth stage or a target of your own.
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Leaf temperature depends on light intensity, the light source, airflow and how fast the leaf is transpiring, so it can sit below or above air temperature. An infrared thermometer pointed at the top of the canopy, or a leaf temperature sensor, gives the figure.
Without leaf temperature, the readout and chart show air VPD. Leaves can sit a few degrees below or above air temperature, so adding a leaf reading gives the VPD at the leaf.
VPD chart
Leaf VPD diagram
The same reading as the sliders above. Water evaporates inside the leaf, where the air spaces are saturated at leaf temperature, and leaves through the stomata into drier air. The bigger the gap in vapor pressure, the harder the pull.
Air at a given temperature can hold a set amount of water vapor, its saturation vapor pressure. Relative humidity is how much of that it holds now. Air VPD is the difference: saturation pressure × (1 − RH/100).
The inside of a leaf is saturated at the leaf's temperature, so the pull on the leaf is the leaf's saturation pressure minus the vapor pressure in the air. When the canopy is 2 °C cooler than the air, leaf VPD is lower than air VPD: about 0.36 kPa lower at 25 °C air, at any humidity. A room run to an air VPD target with cool leaves gives the leaves a lower VPD than the target.
Saturation vapor pressure uses the Tetens equation: 0.6108 × e17.27T / (T + 237.3) kPa, with T in °C.
Any surface colder than the dew point gets wet. Leaves, walls and windows that drop below it at lights-off are where mildew and botrytis start.
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