Vapor Pressure Deficit: Meaning and Calculation
Calculate air VPD and interpret it as one atmospheric driver of water loss rather than a universal crop prescription.
Educational reference · evidence, sources, and limits shown below
Calculate air VPD and interpret it as one atmospheric driver of water loss rather than a universal crop prescription.
Terms to know
- Vapor pressure deficit
- Difference between saturation vapor pressure and actual vapor pressure.
- Air VPD
- VPD calculated with air temperature for both saturation and actual vapor pressure.
- Kilopascal
- Pressure unit commonly used for VPD; 1 kPa equals 1000 pascals.
- Atmospheric demand
- Combined tendency of the environment to support evaporative water loss.
Core science
For air temperature T_air and relative humidity RH, air VPD can be estimated as e_s(T_air) x (1 – RH/100). At the same RH, warmer air produces a larger VPD because saturation vapor pressure is higher. At the same temperature, lowering RH increases VPD.
VPD is a vapor-pressure difference, not a direct measurement of transpiration, stomatal conductance, plant water status, or disease. Radiation, leaf temperature, boundary-layer conductance, stomatal regulation, root supply, and canopy size determine how strongly a plant responds.
A target VPD is therefore a controlled starting envelope tied to stage, genotype, root system, light load, disease pressure, and facility capability. Sudden changes can create transient mismatches because stomata, roots, and environmental-control equipment do not respond instantly.
Why this matters in cultivation
- Use air VPD to compare room air states and anticipate demand, then validate the interpretation with leaf temperature, substrate water use, plant posture, and disease observations. Keep propagation domes, young seedlings, established vegetative plants, and dense flowering canopies as separate control contexts.
- When changing temperature or RH to reach a VPD, evaluate the independent effects of that change. The same VPD can be produced by multiple temperature-RH pairs that differ in heat stress, dew-point risk, pathogen pressure, and equipment load.
Measure and record
Paired inputs
Canopy air temperature, RH, time, location, and sensor IDs.
Calculation
Approved saturation formula, air-VPD equation, units, and software.
Control context
Stage, cultivar, PPFD, irrigation state, canopy density, and CO2 condition.
Response
Leaf temperature, water use, leaf angle, visible stress, and disease observations.
Adjustment
Temperature, humidity, airflow, or irrigation change and verified result.
Common misconceptions
Correction: It describes an air-state gradient and cannot diagnose root supply or plant water status by itself.
Correction: Plant size, root capacity, leaf structure, canopy density, and risk priorities change.
Correction: Temperature, dew point, leaf temperature, and energy load may still differ.
Evidence limits
Published greenhouse VPD ranges are often crop- and system-specific. Cannabis studies remain too limited to support one universal chart across all genotypes, stages, light levels, and facilities.
Related encyclopedia topics
- THC-ENC-082-084, THC-ENC-086-089, THC-ENC-096, and THC-GROW-050.
Source notes
- Michigan State University Extension. Water vapor-pressure deficit.
- Shamshiri R.R. et al. (2018). Review of optimum temperature, humidity and vapour pressure deficit for greenhouse microclimate evaluation and control. International Agrophysics 32:287-302.
- Corredor-Perilla I.C. et al. (2025). Elevated relative humidity significantly decreases cannabinoid concentrations while delaying flowering development in Cannabis sativa L. Frontiers in Plant Science 16:1678142.
- THC Cannabis Plant Science Source Packet v1.1 (project source, May 2026).
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.