Vapor Pressure Deficit (VPD) Calculator
Enter dry-bulb (air) temperature plus one humidity input to compute air VPD, saturation and actual vapor pressure, and optional leaf-to-air VPD — for HVAC condensation control, greenhouse climate management, and food/grain drying.
Inputs
VPD Result
Formula & Calculation Basis
Air VPD and the actual vapor pressure p_w are computed server-side by the PsychroLib v3 API (ProExergy). Leaf-to-air VPD is computed client-side with the Buck (1981) saturation curve, since it requires evaluating saturation pressure at an arbitrary surface temperature not tied to the entered air state.
| Symbol | Meaning | Unit |
|---|---|---|
| T_air | Air (dry-bulb) temperature | °C |
| T_leaf | Leaf / canopy / product surface temperature | °C |
| p_ws(T) | Saturation vapor pressure at temperature T | kPa |
| p_w | Actual (partial) vapor pressure of the air — constant regardless of surface temperature | kPa |
| VPD_air | Air vapor pressure deficit (standard HVAC/meteorology definition) | kPa |
| VPD_leaf | Leaf-to-air VPD — the deficit actually driving transpiration/moisture exchange at the surface | kPa |
Calculation sequence: resolve total pressure → send dry-bulb + chosen humidity value + pressure to the matching PsychroLib endpoint (/calc/from-rel-hum, from-dew-point, from-wet-bulb, or from-hum-ratio) → API returns vapor_pressure_deficit (air VPD) and vap_pres (actual p_w) → if a leaf/surface temperature is entered, compute p_ws(T_leaf) locally and derive VPD_leaf = p_ws(T_leaf) − p_w.
Assumptions & Limits
Included effects
- Ideal-gas moist-air behavior for the air-side state (ASHRAE/Hyland-Wexler formulation via PsychroLib).
- Buck (1981) saturation curve for arbitrary leaf/surface temperature — accurate to within ~0.1% over typical 0–50 °C ranges, and consistent with the ASHRAE curve to well under 1% over that same range.
- Altitude-based standard atmosphere (ISA) when pressure is derived from elevation.
Excluded effects
- Leaf boundary-layer resistance, stomatal conductance, and airflow speed — these govern actual transpiration rate, not just the driving VPD.
- Radiant heating/cooling of the leaf or product surface beyond the entered surface temperature.
- Non-uniform canopy or drying-bed microclimates (this is a point calculation).
Valid input range: approximately 0–50 °C air/leaf temperature for the Buck approximation; relative humidity 0–100%; results become less reliable near or below 0 °C (frost/ice regime) and above ~60 °C.
Engineering Notes
- HVAC: low air VPD (near 0 kPa) signals near-saturation air — a condensation/mold risk on cool surfaces; use alongside dew point to check surface temperatures against building components.
- Greenhouse propagation (unrooted cuttings/clones): target air VPD roughly 0.4–0.8 kPa to limit transpiration stress while cuttings lack roots.
- Greenhouse vegetative growth: target air VPD roughly 0.8–1.2 kPa to balance stomatal opening and nutrient uptake.
- Greenhouse flowering/fruiting: target air VPD roughly 1.0–1.5 kPa; sustained VPD above ~1.6 kPa risks stomatal closure and reduced transpiration/yield.
- Grain/food drying: higher VPD (commonly 1.5–3+ kPa) is desirable to drive moisture removal quickly, but must be balanced against case-hardening or quality loss for heat-sensitive products.
- Leaf-to-air VPD is the more physiologically relevant metric for crop stress than air VPD alone, since leaf temperature commonly differs from air temperature by several degrees.
- Method reference: PsychroLib (ASHRAE Fundamentals 2017, Ch.1) for the air state; Buck, A.L. (1981), "New equations for computing vapor pressure and enhancement factor," J. Applied Meteorology, for the leaf-temperature saturation curve.
- API reference: ProExergy.com.
Revision: v1.0 — initial release, adds application-specific VPD target zones and optional leaf-to-air VPD on top of the PsychroLib v3 air-state calculation.
