VPD Calculator

Two views of the same climate: how strongly the surrounding air can dry a crop, and the vapour-pressure gradient at a measured leaf temperature.

Air temperature and relative humidity together give the actual vapour pressure of the surrounding air: how much water vapour it holds. Both modes of the calculator start from that figure.

Air conditions: Air VPD

Air vapour pressure deficit (Air VPD) compares the actual vapour pressure with the saturation vapour pressure at air temperature. It is calculated from air temperature and relative humidity only, and describes the atmospheric drying demand of the surrounding air. It does not include leaf temperature.

Use it when leaf temperature is unavailable, and to compare greenhouse air conditions between periods, compartments or setpoints. The stage reference ranges apply to Air VPD only, and its zones describe the demand of the air, not the condition of the plant.

• Comparing general atmospheric drying conditions
• Building temperature/RH climate tables
• Monitoring controlled environments where leaf temperature is unavailable
• Comparing climate setpoints between periods or growing areas
• Using broad crop-stage reference ranges cautiously

Plant conditions: plant-to-air vapour pressure difference

Plant-to-air vapour pressure difference compares the saturation vapour pressure at measured leaf temperature with the actual vapour pressure of the surrounding air. It assumes the air spaces inside the leaf are saturated, so it describes the vapour-pressure gradient the plant experiences more closely than Air VPD does. Because leaf and air temperature can differ, the two values can differ meaningfully. It is also described as Vapour Pressure Difference, leaf-to-air VPD or leaf VPD.

One is a deficit of the air, the other a difference between leaf and air. Both are sometimes abbreviated to VPD, but they use different temperatures and are not interchangeable.

Plant-to-air vapour pressure difference is an important atmospheric driving force for transpiration and influences stomatal response. It does not directly measure transpiration rate, stomatal opening or plant stress. Plant response also depends on radiation and leaf energy balance, air movement and boundary-layer resistance, stomatal conductance, irrigation and root-zone water availability, root-zone temperature and oxygen, crop stage and crop load, and plant health and acclimation.

In plant conditions the table keeps the leaf at a constant offset from the air: on every row, leaf temperature is that row’s air temperature plus the offset. A leaf measured 2 °C below 22 °C air is assumed to be 2 °C below the air at 30 °C as well. In practice the offset itself shifts with radiation, airflow and transpiration, so read the table as a what-if around the measured condition.

• Including the plant’s measured temperature
• Understanding how radiation and cooling affect the plant–air gradient
• Comparing different canopy positions
• Investigating condensation risk
• Adding plant-level context to a greenhouse or vertical-farm climate assessment

Plant Empowerment context

Plant Empowerment gives particular attention to Vapour Pressure Difference, which includes leaf temperature: it compares the vapour pressure inside the leaf with that of the surrounding air. On this page that is the plant-to-air vapour pressure difference.

It does not reduce plant activity to one target number. Alongside Vapour Pressure Difference it considers Absolute Humidity and the moisture balance of the greenhouse, the canopy microclimate, radiation and the energy balance, airflow and the boundary layer of still air around the leaf, and it steers on trends rather than single values.

Its published guidance gives two broad indicators: below about 0.2 kPa transpiration slows and the plant becomes less active, and above about 2.0 kPa stomata can begin to close to limit water loss. The calculator uses them to interpret the plant-to-air result, as process indicators rather than crop setpoints.

below 0 kPa

Condensation possible. Surrounding-air vapour pressure exceeds saturation vapour pressure at leaf temperature.

0 to below 0.2 kPa

Very low gradient. Limited outward gradient; transpiration may slow.

0.2 to 2.0 kPa

Transpiration gradient. An outward gradient is present. Actual transpiration depends on stomata, energy, airflow and water availability.

above 2.0 kPa

Very high gradient. Strong evaporative demand; stomatal restriction and water-stress risk may increase.

Broad process guidance based on Plant Empowerment principles, not a universal crop setpoint. Source: Understanding absolute humidity in greenhouse climate control, Plant Empowerment. Vaerens is not affiliated with Plant Empowerment.

Relationship with Humidity Deficit

Air VPD and Humidity Deficit both describe the moisture deficit of the surrounding air. Air VPD expresses the deficit as a pressure difference in kPa. Humidity Deficit expresses it as a moisture quantity, commonly in g/m³ or g/kg. Humidity Deficit is widely used in greenhouse horticulture, while Air VPD is especially common in North American and indoor-growing contexts.

They describe closely related air conditions but are not numerically interchangeable: they use different units and, depending on the Humidity Deficit convention, the conversion also depends on temperature and sometimes atmospheric pressure.

How to measure plant conditions

Measure air temperature and relative humidity as close to the canopy as practical. Measure the temperature of representative, actively transpiring leaves. Conditions at the leaf surface can differ from readings elsewhere in the growing space because of radiation, airflow and the leaf boundary layer.

A handheld infrared leaf-temperature measurement is a snapshot. Repeat measurements at different times, canopy positions and changing conditions when assessing a crop or a climate strategy.

• Measure air temperature and RH at canopy level.
• Use a representative, actively transpiring leaf rather than an exceptional hot or cold leaf.
• Take the air and leaf readings at about the same place and time.
• Repeat measurements across canopy locations and as conditions change.
• Interpret the result alongside radiation, airflow, irrigation and plant observations.

Greenhouses and controlled environments

The same distinction applies in greenhouses, vertical farms, indoor farms and growth chambers. In controlled environments with artificial lighting, leaf and air temperatures may sometimes be closer than in strongly sunlit greenhouse conditions. They should still be treated as separate measurements because transpiration, airflow, lighting, HVAC operation and local humidity can create meaningful differences.