Grow science · 05
Dew point & mold
The water in your air doesn’t leave when the lights go off. It just has less room.
Dew point & night check
Assumes no dehumidifier runs at night – the water stays in the air while the temperature drops. Leaves and buds can be 1–2 °C colder than the air, so keep a margin.
What the dew point is
The dew point is the temperature at which the air is fully saturated. Cool any surface below it and water condenses on it – on tent walls, on leaves, deep inside dense buds.
At 26 °C and 55 % RH the dew point is 16.3 °C. Sounds far away – until you look at the night.
What happens at lights-off
The lamp switches off, the temperature drops, but the moisture stays in the air. Relative humidity climbs with every degree:
| Lights off | Humidity | VPD |
|---|---|---|
| 26 °C (no drop) | 55 % | 1.51 kPa |
| 24 °C | 62 % | 1.14 kPa |
| 22 °C | 70 % | 0.80 kPa |
| 21 °C | 74 % | 0.64 kPa |
| 20 °C | 79 % | 0.49 kPa |
| 18 °C | 90 % | 0.22 kPa |
Same water content as 26 °C / 55 %, calculated with the Magnus formula.
A 5 °C drop turns a perfect flower climate into 74 % humidity. Inside a dense bud, where air barely moves, it’s even wetter – that’s where botrytis starts.
How to stay safe
- Keep the night drop small – 2–4 °C is plenty. A small heater or less exhaust at night helps.
- Dehumidify at night, not just during the day. Put the dehumidifier where the tent pulls its air from.
- Keep air moving around and through the canopy – stagnant pockets are where condensation happens.
- Late flower: aim for under 50–55 % at night.
- Watch the first hour after lights-off – that’s when humidity spikes hardest.
The formula
γ = ln(RH/100) + 17.27·T / (237.3 + T)
Dew point = 237.3·γ / (17.27 − γ)
Same constants as our VPD calculator – and the charts on our shirts.