Explainer: What is Vapor Pressure Deficit (VPD)?

The amount of water in the air can be measured in terms of pressure; the more water there is in the air, the greater the pressure it exerts at the surface. Vapor pressure deficit (VPD) measures how much water is in the air versus the maximum amount of water vapor that can exist in that air, what's known as the saturation vapor pressure (SVP). 

As air warms, its capacity to hold water increases. Thus, its SVP increases as well. The greater the difference between the air’s actual water vapor pressure and its saturation vapor pressure, the more potential it will have to suck moisture from the ground.

VPD is used to measure dryness, or aridity, near the Earth's surface. It is directly related to the rate at which water is transferred from the land surface to the atmosphere. For example, as VPD increases—which means the air is less saturated with water—plants need to draw more water from their roots, which can cause the plants to dry out and die.