Storage Humidity & Vapour Pressure Deficit
Protects fruit
Enter store temperature and relative humidity to get the vapour pressure deficit— how much drier the air is than saturation — so you can hold a low VPD and stop produce shrivel and weight loss.
Set your storage conditions
Next: at 0.12 kPa VPD your produce is in a low-loss zone; raise RH (humidify or seal the room) to push VPD down and slow moisture loss.
Very low VPD (saturated air) slows water loss but can encourage condensation and mould — most cold-stored produce targets high RH (90–98%) without free water on surfaces.
Storage humidity — key facts
- SVP (Magnus)
- 0.6108·exp(17.27T÷(T+237.3))
- VPD
- SVP × (1 − RH)
- High VPD
- dry air → shrivel & weight loss
- Low VPD
- humid air → produce stays fresh
- Most produce
- 90–95% RH (very low VPD)
- Onions/garlic
- drier air, higher VPD
- Cold air
- holds less vapour → lower SVP
- Privacy
- Runs in your browser; nothing uploaded
Keep the air humid so produce keeps its weight
Every fruit and vegetable goes on breathing and losing water after harvest, and the air around it decides how fast. The vapour pressure deficit measures how thirsty that air is: a high VPD pulls moisture out of the produce, causing shrivel, softening and saleable weight loss; a low VPD means the air is near saturation and can barely draw water out. Cold stores are run at high humidity precisely to hold that VPD low.
This tool gives the saturation vapour pressure, the VPD, and the relative humidity and temperature behind them from the conditions you enter. Use it to set a humidity target for each crop, check whether your store's air is too dry, and steer toward a low VPD without tipping into condensation. Pair it with the Cold Storage Shelf-Life and Produce Weight Loss tools for a full storage plan.
Stop shrivel
Hold a low VPD so air can't pull out moisture.
Protect weight
Less transpiration means less saleable loss.
Tune by crop
High humidity for greens, drier for onions.
Avoid condensation
Keep humidity high but temperatures steady.
Frequently Asked Questions
What is vapour pressure deficit (VPD)?+
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.
Why does VPD matter for produce storage?+
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.
How is VPD calculated?+
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.
What VPD should a cold store hold?+
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.
What is saturation vapour pressure?+
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.
How do I lower the VPD in my store?+
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.
Can high humidity cause problems too?+
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.
Does VPD change with temperature at the same humidity?+
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.
Are the figures precise?+
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.