Dew point & condensation risk
Where water will condense in your room, and how far humidity jumps when the lights go off.
Inputs
Wall, window, AC duct or leaf
Result
Dew point
16.3°C
Surface margin
+1.7°C
RH after lights-off
74%
How this is calculated
Dew point
γ = ln(RH/100) + 17.62T/(243.12+T); T_d = 243.12γ/(17.62−γ)
- Magnus formula with Sonntag constants (17.62, 243.12 °C); accurate to about ±0.4 °C between −45 and 60 °C.
Condensation
margin = T_surface − T_dew; water condenses when the surface is at or below the dew point
- The 2 °C 'close' buffer is a safety margin for sensor error (typically ±0.5 °C, ±2–3% RH), not a cited threshold.
Lights-off RH
RH₂ = RH₁ · SVP(T₁) / SVP(T₂) (same absolute humidity; capped at 100%)
- Assumes no moisture is added or removed while the air cools; plants keep transpiring after lights-off, so real RH usually rises more.
Cited targets
From the genetics.guru grow library, in the units each source used. Grades: strong and moderate are peer-reviewed; community practice is untested.
Botrytis high-risk RHflower≥ 70% RHStrong
rapid bud destruction above 70% RH at 17-24 °C
1 source
Botrytis high-risk temperatureflower17–24 °CStrong
in vitro optimum 15-25 °C
2 sources
- Mahmoud M, BenRejeb I, Punja ZK, Buirs L, Jabaji S (2023). Understanding bud rot development, caused by Botrytis cinerea, on cannabis (Cannabis sativa L.) plants grown under greenhouse conditions
- Punja ZK, Ni L (2021). The bud rot pathogens infecting cannabis (Cannabis sativa L., marijuana) inflorescences: symptomology, species identification, pathogenicity and biological control
RH maximum (bud-rot risk)flower≤ 70%Moderate
Botrytis destroys inflorescences rapidly at RH >70% and 17-24 C. Apply to lights-off too.
1 source
air temperature (night)vegetative20–24 °CCommunity practice
Trials ran 22-25 C nights.
2 sources
- Rodriguez-Morrison V, Llewellyn D, Zheng Y (2021-05-11). Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment — Frontiers in Plant Science 12:646020
- Llewellyn D, Golem S, Foley E, Dinka S, Jones AMP, Zheng Y (2022-09-27). Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content — Frontiers in Plant Science 13:974018
air temperature (night)flower18–22 °CModerate
Trial nights 17-25 C. Late-cycle cooling may raise cannabinoids in some cultivars (thesis abstract).
3 sources
- Holweg MM, Curren T, Cravino A, Kaiser E, Kappers IF, Heuvelink E, et al. (2025-02). High air temperature reduces plant specialized metabolite yield in medical cannabis, and has genotype-specific effects on inflorescence dry matter production — Environmental and Experimental Botany 230:106085
- Llewellyn D, Golem S, Foley E, Dinka S, Jones AMP, Zheng Y (2022-09-27). Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content — Frontiers in Plant Science 13:974018
- Westmoreland FM, Kusuma P, Bugbee B (2021-03-23). Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids — PLOS ONE 16(3):e0248988