Vapor Pressure Deficit (VPD)

VPD air is the room's saturation pressure less the saturation pressure at its dew point; VPD leaf is the leaf's saturation pressure less the vapour pressure already in the air. Saturation pressure uses the ASHRAE Fundamentals correlation, over ice below 32 °F and over water above it. Both results carry their unit, kPa, in the value.

Enter relative humidity as a percentage: 60 means 60%.

As a leaf grows it cools itself by evaporating, so the leaf runs cooler than the room. A common rule of thumb is about 2 °F below room temperature when you have not measured it.

Altitude is fixed at sea level, 14.696 psia, as it is on the published calculator.

The results are blank when the temperature they need has been left out, and the low-temperature warning appears once the room is below freezing, where the correlation is least reliable.