Work out vapour pressure deficit instantly with clear inputs, formula shown and shareable results.
Vapour pressure deficit is the gap between saturation and actual vapour pressure, and it drives transpiration far more directly than relative humidity does. Leaves usually sit a couple of degrees below air temperature, which lowers their saturation pressure and the deficit they experience. Growers target 0.8-1.2 kPa leaf VPD for most crops.
Vapour pressure deficit
VPD = es(leaf temperature) - ea(air)
Actual vapour pressure
ea = RH / 100 x es(air temperature)
Because transpiration is driven by the deficit at the leaf surface, and transpirational cooling keeps leaves below air temperature.
Stomata close to conserve water, which stops carbon dioxide uptake and halts growth even when light and nutrients are ample.