Storage Humidity & Vapour Pressure Deficit

VPD is how much drier the air is than saturation — the gap between the saturation vapour pressure at a given temperature and the actual vapour pressure in the air. A high VPD means the air is thirsty and pulls moisture out of stored produce; a low VPD means the air is near saturation and produce loses little water.

Fresh produce keeps respiring and transpiring after harvest. When the surrounding air has a high VPD it draws water out of the produce, causing shrivel, softening and weight (saleable) loss. Cold stores are deliberately held at high humidity — a low VPD — so the air can't pull much moisture out and produce stays plump and fresh.

First find the saturation vapour pressure (SVP) at the store temperature using the Magnus formula: SVP = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) kPa. The actual vapour pressure is SVP × relative humidity. VPD = SVP − actual vapour pressure = SVP × (1 − RH). The calculator does all of this from the temperature and humidity you enter.

Most fresh fruit and vegetables store best at 90–95% relative humidity, which at cold-store temperatures is a very low VPD (often under 0.1 kPa). Leafy greens and root crops want the highest humidity; onions, garlic and pumpkins prefer drier air with a higher VPD. The right target depends on the crop's skin and how prone it is to shrivel.

Saturation vapour pressure (SVP) is the maximum water vapour the air can hold at a given temperature — it rises steeply as the air warms. Cold air holds far less vapour than warm air, which is why even a small temperature drop in a store can push the air toward saturation and form condensation on produce or coils.

Raise the relative humidity (humidifiers, wet floors, misting, or simply a full store of moist produce) and keep the temperature stable. Avoid oversized refrigeration that runs in short hard bursts — large coil temperature swings dry the air. Good insulation, gentle airflow and a small temperature difference between the air and coils all keep VPD low.

Yes — air held too close to saturation (VPD near zero) can lead to condensation, free water on surfaces and fungal rots if temperatures fluctuate. The aim is high but controlled humidity, with steady temperature, so the VPD stays low enough to stop shrivel but not so low that water condenses on the produce.

Yes. Because saturation vapour pressure rises with temperature, the same relative humidity gives a higher VPD in a warm room than a cold one. That's one reason cold storage helps: chilling the air lowers SVP, so even moderate humidity gives a small VPD and slow moisture loss.

They're solid planning figures based on the standard Magnus equation for saturation vapour pressure. Real stores vary with airflow, coil temperature, door openings and produce load. Use a calibrated hygrometer, hold steady temperatures, and treat the VPD here as a target to manage toward rather than an exact prediction of moisture loss.