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Vapour-Pressure Deficit

The pressure difference between saturation vapour pressure at air temperature and actual atmospheric vapour pressure.

Version
v1 · 2026-09-28 · History
Domain-specific #
12762
Domain group
Natural Sciences
Origin domain
Geology & Earth Sciences
Subdomains
Atmospheric Moisture, Meteorology → Geology & Earth Sciences
Aliases
Vapor pressure deficit, VPD, Saturation vapour pressure deficit

Core Idea

Vapour-pressure deficit is saturation vapour pressure at air temperature minus actual vapour pressure, expressing the atmosphere's drying demand relative to saturation in pressure units.

At a known air temperature, subtract measured actual vapour pressure from the corresponding saturation pressure. When actual equals saturation pressure, VPD is zero and condensation becomes possible.

Structural Signature

Sig role-phrases:

  • Air temperature — Determines saturation pressure. It is input. Counterfactual: Relative humidity without temperature cannot fix VPD.
  • Saturation vapour pressure — Sets the upper moisture pressure. It is reference. Counterfactual: Using leaf temperature changes the quantity.
  • Actual vapour pressure — Represents existing water vapour. It is state. Counterfactual: Missing it prevents the deficit.
  • Subtraction — Computes e_s(Ta)-e_a. It is relation. Counterfactual: A ratio would be relative humidity.
  • Pressure unit — Expresses kPa or equivalent. It is measure. Counterfactual: Percent units signal another metric.
  • Plant–air gradient — Connects atmospheric VPD to transpiration with additional leaf conditions. It is application. Counterfactual: VPD alone does not determine stomatal response.

What It Is Not

  • It is not relative humidity.
  • It is not absolute humidity.
  • It is not always the leaf-to-air pressure difference.
  • It is not a direct transpiration rate.
  • Closest near-miss. Relative humidity describes a ratio and can change with temperature even at the same actual vapour pressure.

Scope of Application

  • Greenhouses. Controls atmospheric demand.
  • Plant physiology. Relates to stomatal response.
  • Meteorology. Describes saturation deficit.
  • Drying engineering. Tracks evaporation potential.

Clarity

Include nonnegative differences between saturation pressure at stated air temperature and actual vapour pressure. Exclude relative humidity percentages, dew-point depression, leaf-to-air VPD without leaf temperature, and moisture content alone.

Manages Complexity

High VPD can increase evaporative demand while stomata may close and limit flux. Atmospheric VPD and leaf-to-air gradient use different temperatures.

Abstract Reasoning

  1. Air temperature — Determines saturation pressure. Relative humidity without temperature cannot fix VPD.
  2. Saturation vapour pressure — Sets the upper moisture pressure. Using leaf temperature changes the quantity.
  3. Actual vapour pressure — Represents existing water vapour. Missing it prevents the deficit.
  4. Subtraction — Computes e_s(Ta)-e_a. A ratio would be relative humidity.
  5. Pressure unit — Expresses kPa or equivalent. Percent units signal another metric.
  6. Plant–air gradient — Connects atmospheric VPD to transpiration with additional leaf conditions. VPD alone does not determine stomatal response.

Knowledge Transfer

The saturation-minus-actual pressure relation transfers across plant, atmosphere, and drying studies after air temperature and moisture measures are aligned; relative humidity alone cannot substitute without temperature.

Examples

Applied / In Practice

At a known air temperature, subtract measured actual vapour pressure from the corresponding saturation pressure.

Mapped back: reference → air saturation; state → actual; output → kPa.

Applied / In Practice

When actual equals saturation pressure, VPD is zero and condensation becomes possible.

Mapped back: difference → 0.

Structural Tensions

T1 — Drying Demand versus Plant Regulation. High VPD can increase evaporative demand while stomata may close and limit flux.

Diagnostic: Are biological controls modeled?

T2 — Air Reference versus Leaf Reference. Atmospheric VPD and leaf-to-air gradient use different temperatures.

Diagnostic: Which surface sets saturation?

Structural–Framed Character

Determines saturation pressure. Sets the upper moisture pressure. High VPD can increase evaporative demand while stomata may close and limit flux.

Structural Core vs. Domain Accent

Expresses kPa or equivalent. Connects atmospheric VPD to transpiration with additional leaf conditions. The identity changes when saturation is evaluated at another surface temperature without relabeling.

This entry presupposes Subtraction.

  • Approved root. The frozen graph retains vapour-pressure deficit without a parent edge.

  • Related — Relative humidity and Dew point. A saturation ratio. A saturation temperature.

Relationships to Other Abstractions

Local relationship map for Vapour-Pressure DeficitParents appear above the current abstraction, mutual partners to the right, and children below. Node labels state whether each abstraction is prime or domain-specific; colors identify relation types.Vapour-PressureDeficitDOMAINDomain-specific abstraction: Subtraction — presupposesSubtractionDOMAIN

Current abstraction Vapour-Pressure Deficit Domain-specific

Parents (1) — more general patterns this builds on

  • Vapour-Pressure Deficit presupposes Subtraction Domain-specific

    Vapour-Pressure Deficit presupposes Subtraction because the quantity is defined as saturation vapour pressure minus actual vapour pressure.

Hierarchy path (1) — routes to 1 parentless root

Neighborhood in Abstraction Space

Vapour-Pressure Deficit sits in a moderately populated region (52nd percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.

Family — Analytical Measurement & Thermal Properties (27 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-08

Not to Be Confused With

  • Relative humidity. Tell: A saturation ratio.
  • Dew point. Tell: A saturation temperature.
  • Specific humidity. Tell: Mass mixing measure.
  • Leaf VPD. Tell: May use leaf temperature.

References

  • Frozen Wikipedia discovery revision: https://en.wikipedia.org/wiki/Vapour-pressure_deficit (revision 1368628666).
  • Preserved source candidate: https://doi.org/10.5194/essd-2022-212
  • Preserved source candidate: https://journals.ametsoc.org/view/journals/apme/54/6/jamc-d-14-0321.1.xml
  • Preserved source candidate: http://www.ecaa.ntu.edu.tw/weifang/class-cea/Greenhouse%20Condensation%20Control%20VPD,%20AEX-804-01.htm
  • Preserved source candidate: https://web.archive.org/web/20210119031221/http://www.ecaa.ntu.edu.tw/weifang/class-cea/Greenhouse%20Condensation%20Control%20VPD,%20AEX-804-01.htm
  • Preserved source candidate: https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.16485
  • Preserved source candidate: https://www.greenhouse-management.com/greenhouse_management/managing_vapor_pressure_deficit_greenhouse_crops/vapor_pressure_deficit_greenhouse_crops.htm
  • Preserved source candidate: https://wildfiretoday.com/the-role-of-vapor-pressure-deficit-in-wildland-fires/

The frozen Wikipedia revision is discovery provenance. The retained source set was reviewed for identity, formal or operational relation, and scope. The encyclopedia's structural synthesis is bounded to those claims; a thin authority surface is recorded as a nonblocking source-strengthening repair rather than concealed.