Vapor Pressure Deficit: Atmospheric Dryness Metric
Vapor pressure deficit (VPD) quantifies the difference between the saturation vapor pressure at a given air temperature and the actual vapor pressure of water vapour. Expressed in kilopascals, millibars, or hectopascals, VPD represents the drying power of the air—higher values correspond to greater evaporative demand on plants, soils, and water bodies. This article defines VPD, recounts its measurement history, examines conceptual tools, and highlights applications in agriculture, building design, and fire weather.
Use this overview with the relative humidity explainer to contrast common humidity metrics, and leverage the dew point calculator to derive actual vapor pressure when computing VPD.
Definition and Psychrometric Relationships
Formula and units
VPD equals the saturation vapor pressure es(T) minus the actual vapor pressure ea: VPD = es − ea. Saturation pressure derives from temperature via formulations such as Tetens’ equation. Actual pressure can be obtained from dew point or relative humidity (ea = RH × es). Units typically use kilopascals; horticulture often works in kilopascals or millibars.
Relationship to relative humidity
While relative humidity expresses the ratio RH = ea / es, VPD captures the absolute gap between saturation and current vapour levels. Thus, two environments with the same RH but different temperatures can have drastically different VPD, influencing transpiration.
Psychrometric chart interpretation
On a psychrometric chart, VPD corresponds to horizontal distance between a state point and the saturation curve. Monitoring this distance helps greenhouse managers maintain target humidity and temperature setpoints.
Historical Development
Early plant physiology
VPD concepts emerged in early twentieth-century plant physiology as researchers like D.S. Johnson investigated transpiration drivers. By the 1940s, controlled environment studies routinely reported VPD alongside temperature and light conditions.
Meteorological adoption
Fire weather specialists adopted VPD in the 1960s to characterise atmospheric drying power influencing fuel moisture. Agencies now map VPD anomalies to flag wildfire risk.
Modern sensor technology
Contemporary data loggers compute VPD automatically from humidity and temperature sensors, integrating with irrigation controllers, HVAC systems, and precision agriculture dashboards.
Applications and Importance
Crop management
Growers adjust irrigation, ventilation, and shading based on VPD targets specific to crop species. High VPD can stress stomata, while low VPD increases disease risk. Combining VPD tracking with the pool evaporation calculator demonstrates how evaporative demand scales with environmental conditions.
Building climate control
HVAC designers consider VPD when evaluating occupant comfort and condensation risks. Integrating VPD into controls can balance dehumidification loads with energy efficiency goals.
Wildfire forecasting
Elevated VPD dries fuels rapidly, serving as an input to fire danger indices. Climate scientists correlate VPD anomalies with large fire seasons, leveraging tools like the greenhouse gas unit converter to place emissions and climate drivers in consistent units.