How this instrument works
Vapor pressure deficit, VPD, measures the gap between two quantities: saturation vapor pressure (SVP), the maximum water vapor pressure air could hold at a given temperature before it's fully saturated, and actual vapor pressure (AVP), the water vapor pressure the air is actually holding right now. That gap is the 'pulling power' driving transpiration and evaporation — a large deficit pulls moisture out of leaves and soil quickly, while a deficit near zero means the air is nearly saturated and barely pulling at all.
SVP depends only on temperature, and it does not rise in a straight line — the relationship is exponential, following the Tetens equation documented in FAO Irrigation and Drainage Paper No. 56, the standard international reference for crop water-use calculations. Warmer air can hold dramatically more water vapor before saturating than cooler air can, which is why the same relative humidity reading means something very different at 20°C than it does at 30°C, and why VPD (which folds temperature and humidity into one pressure figure) gives growers a more consistent signal than relative humidity alone.
That's the practical reason greenhouse and grow-room operators track VPD rather than RH. Two rooms both reading 60% relative humidity can have meaningfully different actual drying power on a plant if one is warmer than the other, because SVP — and therefore how much moisture the air can still absorb — differs sharply between them. VPD collapses temperature and humidity into a single number in kilopascals that behaves consistently across conditions, which is why growers commonly reference a target VPD range for a given crop stage rather than a target humidity percentage.
- Enter the current air temperature into Air temperature (°C).
- Enter the current relative humidity reading into Relative humidity (%).
- Read Saturation vapor pressure, SVP (kPa) — the maximum vapor pressure the air could hold at that temperature if fully saturated.
- Read Actual vapor pressure, AVP (kPa) — what the air is actually holding right now, computed from SVP and the entered humidity.
- Read Vapor pressure deficit, VPD (kPa) — the gap between the two, and the figure most commonly compared against a crop's target range.
Worked example — 25°C air at 60% relative humidity
Air at 25°C and 60% relative humidity is a common upper-range greenhouse reading. Enter 25 into Air temperature (°C) and 60 into Relative humidity (%). The exponent works out to 17.27 x 25 / (25 + 237.3) = 431.75/262.3 = 1.64602, and e raised to that power is 5.18628, so Saturation vapor pressure, SVP (kPa) reads 3.1678 — 0.6108 times 5.18628.
Actual vapor pressure, AVP (kPa) is SVP scaled by the humidity fraction: 3.1678 x 0.60 = 1.9007. Vapor pressure deficit, VPD (kPa) is the difference between the two: 3.1678 - 1.9007 = 1.2671 kPa — a reading that sits at the upper edge of the roughly 0.8-to-1.2 kPa range commonly cited as ideal for the propagation-to-vegetative growth stages of many greenhouse crops.
Questions
What is vapor pressure deficit and why not just use relative humidity?
VPD is the gap between how much water vapor the air could hold at its current temperature (SVP) and how much it actually holds (AVP). Relative humidity alone can be misleading because the same percentage means very different actual drying power at different temperatures — 60% RH at 20°C and 60% RH at 30°C pull moisture from a leaf at noticeably different rates, which is why VPD is the figure growers typically track instead.
What do SVP and AVP actually represent?
SVP, saturation vapor pressure, is the ceiling — the maximum vapor pressure the air could hold at its current temperature before becoming fully saturated. AVP, actual vapor pressure, is what the air is holding right now, calculated as SVP scaled down by the humidity percentage. VPD, their difference, is the remaining 'room' the air has to absorb more moisture, which is what drives transpiration.
What's considered a healthy VPD range for greenhouse crops?
Roughly 0.8 to 1.2 kPa is commonly cited as an ideal range for the propagation-through-vegetative growth stages of many crops, though the exact target shifts somewhat by species and growth stage — flowering and fruiting stages often tolerate or benefit from a slightly higher deficit. Treat any single 'ideal number' as a starting range to fine-tune against your specific crop's response rather than a fixed target.
Why does SVP rise so sharply with temperature?
Because the underlying Tetens formula is exponential, not linear — SVP = 0.6108 x exp(17.27T / (T+237.3)). Warm air can hold dramatically more water vapor before saturating than cool air can, so a modest temperature increase produces a disproportionately larger jump in the ceiling the air could still absorb, and therefore in VPD as well if humidity doesn't rise to match.
Is this the same SVP formula used in reference evapotranspiration calculations?
Yes — the Tetens saturation-vapor-pressure equation used here comes directly from FAO Irrigation and Drainage Paper No. 56, the standard international reference for computing crop evapotranspiration and irrigation water requirements. VPD and reference ET calculations share this same SVP step, since both ultimately depend on how much 'pulling power' the surrounding air has.