Vapour-pressure deficit

Vapour pressure-deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated.

It is this last instance that makes VPD important for greenhouse regulation.

If a film of water forms on a plant leaf, it becomes far more susceptible to rot.

On the other hand, as the VPD increases, the plant needs to draw more water from its roots.

As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa.

[citation needed] In ecology, it is the difference between the water vapour pressure and the saturation water vapour pressure at a particular temperature.

Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.

Saturation pressure can be looked up in a psychrometric chart or derived from the Arrhenius equation; a way to compute it directly from temperature is where To convert between Rankine and degrees Fahrenheit:

We then can compute the partial pressure of the water vapour in the air by multiplying by the relative humidity [%]: and finally VPD using

[3][4] The vapour pressure deficit can be utilized when predicting behaviour of a wildfire.