What is VPD?

What is VPD?

VPD stands for Vapor Pressure Deficit. To understand VPD we must first understand the concept of vapor pressure and how it relates to humidity.

Most of us are familiar with the concept of relative humidity - which is the ratio of actual water in the air to the theoretical saturation point at the current air temperature, expressed as a percentage. The saturation point is the maximum amount of water vapor that an air mass can hold at a given temperature and pressure. Anything in excess of the saturation point will condense out as liquid water. Therefore, whenever you see condensation or ‘dew’ on the surface of an object, it indicates that the air immediately surrounding that surface has become cooled to below its saturation point, causing some of the water vapor to condense into its liquid form. The saturation point is also called ‘dew point’.

What does pressure have to do with humidity?

Vapor pressure is a different way of thinking about humidity. Air typically contains about 1-4% water vapor. All gasses in the air exert a pressure. The combination of theses gasses including water vapor produce a pressure at sea level of about 1013 mb (14.7 psi). Within this total, water vapor pressure accounts for about 2 mb (millibars) under extremely dry and cold conditions to about 42 mb of pressure at 30 degrees Celsius and 100% relative humidity.

VPD is a measure of the difference (or deficit) between the pressure exerted by the moisture currently in the air and the pressure at saturation. VPD units are most often expressed in standard pressure units such as millibars, kilopascals, or pounds per square inch. VPD is sometimes expressed in mass deficit concentration units such as grams of water per cubic meter of dry air, or grams of water per kilogram of dry air. The Argus system uses millibars as the default measurement unit although other measurement units can be configured.

Why not just use Relative Humidity?

In many situations you can. At any fixed air temperature and pressure, there is an excellent inverse relationship between RH and VPD. For this reason, many growers simply use RH values for the same purposes with good results. However, if the air temperatures change significantly, VPD can provide a more accurate indication of the current evaporation potential since it combines the effects of both temperature and humidity into a single value.

More information on VPD can be found below which includes a link to how one of Argus' clients, Qualitree Propagators, is using VPD to help them optimize their crop.