Calculator
Formula Used
- Dew point (Magnus): Td = (b·γ)/(a−γ), where γ = ln(RH/100) + (a·T)/(b+T), using a=17.625, b=243.04 (T, Td in °C).
- Condensation margin: Margin = Td − Ts. If Margin ≥ 0, condensation is possible on that surface.
- VPD: VPD = es(T) − e, where e = RH·es/100. Reported in kPa for garden decision-making.
- Absolute humidity: AH = 216.7·e/TK (g/m³), with e in hPa and TK in Kelvin.
- Optional surface estimate: a simple interpolation between indoor and outdoor temperature based on cover type. Use measured surface temperature when you can.
How to Use This Calculator
- Measure air temperature and humidity near plant height.
- Measure the coldest surface temperature you care about.
- If you cannot measure, enable surface estimation and add outdoor temperature.
- Click Calculate Risk and review the risk level and margin.
- Use the recommendations to adjust ventilation, watering, and insulation.
- Export your results using the CSV or PDF buttons for records.
Example Data Table
| Scenario | Air Temp | RH | Surface Temp | Likely Outcome |
|---|---|---|---|---|
| Cool night, humid greenhouse | 20 °C | 90% | 15 °C | High risk; cover and leaves may wet. |
| Mild conditions with airflow | 22 °C | 70% | 18 °C | Low to moderate risk; monitor at dusk. |
| Warm, drier day | 28 °C | 55% | 26 °C | Unlikely; keep steady ventilation. |
Professional Guide to Condensation Risk in Gardens
1) Why dew point matters under covers
Condensation forms when a cold surface drops to the air dew point. In covered gardens, glass, film, and dense foliage cool quickly after sunset. When air remains humid, water vapor turns into droplets on the cover and on leaves. This wetness increases disease pressure and reduces light transmission the next morning.
2) Interpreting the condensation margin
The calculator reports a margin of Td − Ts. A positive margin means the surface is colder than the dew point, so condensation is likely. For example, at 20 °C and 90% RH, dew point is about 18.3 °C. If a glass panel is 15 °C, the margin is roughly +3.3 °C, which is a high-risk situation.
3) Useful operating ranges for growers
Many ornamentals and vegetables perform best when night leaf wetness is minimized. A practical target is to keep the margin below 0 °C, or at least within −2 °C to +0 °C for short periods. The displayed VPD helps verify drying potential: values near 0.3–0.8 kPa commonly indicate gentle drying, while very low VPD signals persistently damp air.
4) Data-driven adjustments that work
If risk is high, lower humidity before the temperature drop. Brief venting can remove moisture, and circulation fans reduce cold pockets. Watering in the morning prevents late-day evaporation spikes. If covers run cold, add thermal screens, seal drafts, or gently heat air near the glazing. Recheck after each change to confirm improvement.
5) Logging results for repeatable routines
Exporting CSV or PDF builds a simple record of conditions, actions, and outcomes. Track dates with recurring issues, such as clear nights or heavy irrigation days. Over a few weeks, you can identify thresholds that trigger condensation in your structure and standardize ventilation and watering schedules accordingly.
FAQs
1) What does “High” risk mean here?
It means the dew point is at least 2 °C above the surface temperature, so visible droplets are likely. Prioritize lowering humidity and warming or insulating the cold surface.
2) Should I use leaf temperature or cover temperature?
Use the coldest surface that can get wet. For disease control, leaf temperature is most relevant. For dripping and light loss, use the cover surface.
3) Why does risk increase at night even if RH stays similar?
Surfaces cool faster than the air after sunset. If the surface temperature drops toward the dew point, condensation starts even without a major RH change.
4) Is the surface estimate accurate enough?
It is a simplified approximation based on cover type and indoor/outdoor difference. For best accuracy, measure surface temperature with an infrared thermometer and keep estimation off.
5) How can I reduce humidity quickly?
Vent briefly when outside air is drier, run circulation fans, avoid evening watering, and remove standing water. Even small humidity reductions can move the dew point away from cold surfaces.
6) What VPD value should I aim for?
There is no single value for all crops, but very low VPD indicates damp air. Many growers target a modest night VPD (around 0.3–0.8 kPa) to limit leaf wetness without excessive drying.
7) Can condensation occur when margin is slightly negative?
Yes, because temperatures fluctuate and the coldest spot may be cooler than your measurement. Treat margins near 0 °C as a warning and improve mixing, insulation, or humidity control.